There are five 3-0 teams in the NFL. Do you know what they have in common? It's not All-Pro quarterback play. For all the emphasis we put on the importance of finding a truly great leader under center, the quarterbacks who have started for those undefeated teams are Joe Flacco, Trevor Siemian, Carson Wentz, Sam Bradford, Shaun Hill, Jimmy Garoppolo and Jacoby Brissett. That's a group that includes four passers making their first NFL starts this season, a guy coming off of a torn ACL, a career backup and a quarterback who was traded one week before the season started.

Yes, those quarterbacks have played above expectations as a group, but there's a more obvious factor influencing our perception of their play and linking the teams: their defenses.

Four of the five undefeated teams rank among the five stingiest defenses in the league so far this season in terms of points allowed, with the fifth team, the Broncos, in eighth. The defending champs are the most recent example of how a team can overcome middling quarterback play to win a Super Bowl, as they did in 2015. When they get above-average play from their passer, as the Broncos did from Siemian on Sunday, great defenses can make even really good teams like the Bengals look like they don't belong.

Who has the best defense in football right now? It's tough to say, if only because three weeks isn't a whole lot of data. Last year, the teams who were tops in the league through three weeks were generally among the best in football, but there was a lot of shuffling to be done. The Seahawks, who finished with the league's best scoring defense, were 10th after three games. They were just ahead of the Eagles in 11th, and Philly ended up finishing with the fifth-worst defense in the league.

Let's try to run through the best defenses in the league, then, with the numbers from the first three weeks of 2016 and a few other useful dollops of information. (Week 3 isn't over, but let's just say Atlanta and New Orleans aren't near this list.) What each team has done in 2016 is the most recent and important information, but this also will consider how they performed in 2015, the sustainability of that performance, and the quality of the opposition they've faced so far this season. There's no bad spot on this list: Every one of these defenses is great. After seeing a run of dominant performances on Sunday, though, it seemed like a good idea to take stock of where the best of the best are at after Week 3.

Fletcher Cox and Richard Sherman have helped their defenses get off to hot starts this season. USA TODAY Sports, AP Photo

This is almost entirely for what the Cardinals did last year and in how they made Jameis Winston look like a high-school quarterback in Week 2. It's certainly not for Sunday, when a moribund Bills offense without Sammy Watkins ripped them to shreds and ran for 208 yards. The pass rush was still around, as the Cardinals sacked Tyrod Taylor four times and knocked him down eight times in 29 dropbacks, but the run defense wasn't. While the Cardinals took down the Bills for negative yardage four times on the ground, Buffalo had two touchdown runs of 20 yards or more to go with a 49-yard run by Taylor.

While the Bills were a relatively unimpressive 4-of-13 on third down on Sunday, third downs have been the notable problem for the Cardinals this year. They're struggling to get off the field. Last year, teams converted on 35.7 percent of their third-down tries against Arizona, which was the eighth-best third-down defense in the league. This year, the Cards are up at 45.2 percent, which is the sixth-worst rate in the league. That alone isn't a justification for failing -- there are three teams in this top 10 with worse rates -- but it's a start for their problems.

Arizona's secondary is still a work in progress. Brandon Williams was a mess against the Patriots in Week 1 before giving way to Marcus Cooper part of the time in Week 2 and ceding his starting job this past week. Tyrann Mathieu, who was so valuable in 2013 and 2015 as a slot corner, isn't fully recovered from a torn ACL and is playing a vanilla free safety role while he plays through pain. They're not scaring receivers right now, as evidenced by ESPN Stats & Information's estimate that Cardinals opponents have dropped just 0.9 percent of passes this year, the lowest rate in the league. There are other factors -- the defense gets the scoring blame for Buffalo's blocked field goal and subsequent touchdown return yesterday -- but given that the Cards were expected to scoot through their early-season slate, this is a disappointing start from a defense that can be great. They should improve as the season goes along.

The Panthers have faced Siemian, Blaine Gabbert and Bradford this year. They've allowed 70 points, which goes down to 64 if you ignore Minnesota's punt return touchdown from Sunday. The Panthers are allowing only about four additional points per game versus their sixth-best scoring defense from a year ago, but they're failing to dominate teams who are missing players at key positions and whose offenses are charitably works in progress.

Luke Kuechly and Thomas Davis are leading a defense that is breaking in some new cornerbacks. AP Photo/Bob Leverone

The problem, in part, is the offense putting Carolina in tough situations. Last year, the Panthers' offense turned over the ball just 19 times in 16 games, the eighth-best rate in the league. This year, through three weeks, they've given away the ball eight times. The eight subsequent offensive drives after those giveaways started with an average of 40 yards to go for an opposing touchdown and produced a total of 30 points, nearly half of the total Carolina has allowed on defense this year. The defense's average starting field position is the seventh-worst figure in the league, with 68.7 yards to go for an opposition score.

Left to their own devices, the Panthers have been good enough on defense. They've produced three-and-outs on a league-high 51.3 percent of opposing drives this season. Teams are converting on only 33.3 percent of their third downs, the sixth-best rate in the NFL. Their red zone defense has been effective. But this was a unit that thrived on takeaways last year, snatching the ball away on a league-high 19.4 percent of opposition drives. This year, they're in ninth in the same category, at 15.4 percent. That's not a huge difference, but these are the sorts of passers the Panthers should be feasting on. If they're merely a pretty good defense against the likes of Siemian and Gabbert, their secondary will need to look a lot better against the Matt Ryans and Drew Breeses to come.

Baltimore is back. Nobody will argue that they've played threatening competition, given that the Ravens have faced the Bills, Browns and Jaguars in three weeks, but the Ravens have allowed just 44 points total. Their run defense has generally been excellent; they did allow one 85-yard touchdown run to Isaiah Crowell (not good), but the Ravens have held the other 67 rushing attempts they've faced to a total of 173 yards, or less than 2.6 yards per carry (actually great).

The new Ravens are built less around a pair of dominant edge rushers and more around the interior of their defense, where they're strong up the middle. Nose tackle Brandon Williams has to be one of the most underrated players in football, while the breakout season that seemed to be in the cards for Timmy Jernigan in 2015 has begun to unspool this year. Jernigan has three sacks and six hits through three games. C.J. Mosley continues to show off his range as one of the best cover linebackers in the league, while Eric Weddle has solidified things for the Ravens in the backfield since coming over from San Diego.

Weddle is directing traffic, but this is a disciplined, well-coached defense all around. The Ravens don't have a great secondary, but they're often in the right place and have been allowing a league-low 3.46 yards after catch per reception. When the defense gets a hand on running backs, the play stops; Baltimore is allowing 0.99 yards after contact on the ground, the lowest rate in the NFL. They have the second-fewest defensive penalties in the league (four) through three games.

To be fair, their turnover numbers are a little inflated. The Ravens have five picks over three games, but two of them were desperation throws on the final drives of their last two contests. On the other hand, they've also forced three fumbles on defense and failed to recover any of them, which is unlikely to recur. They're still a little rough around the edges, and you would worry about their cornerbacks against top-level receivers, as Dean Pees chose to shadow Allen Robinson with Shareece Wright on Sunday and saw his No. 2 corner walked around the field as a result. With the Raiders, Washington, Giants, Jets and Steelers in store for Wright & Co. over the next five weeks, we're about to see if they can match up against all kinds of quality wideouts.

Week 3 wasn't exactly kind to the Texans, given they were shut out 27-0 and allowed the Patriots to run for 185 yards and three touchdowns while getting dominated up front. Looking at the bigger picture, though, the Houston front seven has been a force to be reckoned with for most of 2015 and the first couple of games in 2016. They have the league's second-best sack rate this year at 10.1 percent, and that's with J.J. Watt at less than 100 percent. He has 1.5 sacks and five quarterback knockdowns and doesn't lead the team in either category; that honor belongs to rotation rusher John Simon, who has 2.5 sacks and six hits after racking up six more hits in the final three games of 2015. (He does not lack for style, either.)

The secondary has generally been good in coverage, but it could help the cause by creating more takeaways. The Texans were 12th in interception rate last year but have just one INT in three games to start 2016. They also need to wrap up as tacklers, as opposing rushers are averaging 2.1 yards after contact, the second-worst rate in the league. Only the Falcons have been worse. In all, though, the Texans have been very sound on defense; 45.6 percent of opposing series by the offenses facing Houston have failed to produce a first down or a touchdown, which is the highest rate in the league so far. A year ago, they were third in the same category.

The Chiefs were ripped apart for 21 points by Philip Rivers in the first half of Week 1. In the ensuing five halves of football they've played, the Chiefs have allowed a total of 28 points, including just one touchdown. They've generated 10 takeaways, eight of which came in a stunning performance against the Jets and Ryan "Sixpickrick" on Sunday, producing the league's second-best takeaway rate (25 percent). Given that they picked off Fitzpatrick three times in the red zone, you probably won't be surprised that Kansas City's red zone numbers are great: They've allowed an average of three points per trip inside the 20, the second-best rate in the league.

The Chiefs have been really good defensively for five of six halves, and Justin Houston isn't even on the field. Steve Flynn/USA TODAY Sports

Those numbers sound like the Chiefs are a fluke driven by one game of madness, but that's not fair. Over the past 17 weeks of regular-season football, the Chiefs have allowed just 15.4 points per game while contributing 2.7 points per game on defense, both of which are the best rates in football. The 38.3 QBR they've allowed over that time frame is also the league's best. Opposing passers are last in completion percentage, next-to-last in yards per attempt, and last in passer rating by nearly seven full points at 65.8. That's roughly how current Chiefs backup Nick Foles performed last year for the Rams.

Bob Sutton's defense doesn't get to play Fitzpatrick every week, so the 25 percent turnover rate will regress. On the flip side, the Chiefs also have faced the league's third-worst average starting field position this year (just 67.5 yards to go for a touchdown) after inheriting the league's third-best field position in 2015; given the nature of their no-giveaways offensive style, they're likely to face longer fields on defense as the year goes along.

The one truly concerning thing with Kansas City, though, is the missing pass rush. With Justin Houston on the shelf, the Chiefs have a mere three sacks in three games and have pressured opposing quarterbacks on just 14.4 percent of their dropbacks this season. Only the Steelers, at 12.8 percent, have been worse. If the pass rush doesn't arrive soon, when the turnovers begin to disappear, the Chiefs are going to be stuck on the field for some long defensive drives.

The Eagles were a respectable 17th in defensive DVOA last year, finishing 28th in points allowed because they were a Chip Kelly defense and faced 194 drives, trailing only the Broncos. And everyone suspected that the Eagles were likely to improve with Jim Schwartz's arrival, given how he had been able to turn around the defenses of the Lions and Bills in recent years.

But this? This is insane. The Eagles have allowed 27 points in three games. They allowed as much in eight different games a year ago. This is tied for the 18th-best start on defense since 1990. And while you could have pointed to the quality of the opposition with their wins over the Browns and Bears, there's no such argument with the Eagles holding the Steelers to three points. Washington couldn't shut down the Steelers with Josh Norman; Philly's top cornerback is probably Nolan Carroll, and while Antonio Brown got his 140 yards, the rest of the offense produced a grand total of 146 yards from scrimmage.

As was the case in his previous stops, Schwartz has managed to take the heat off of his secondary by creating pressure without blitzing. The Eagles blitz on just 15.0 percent of opposing dropbacks, the second-lowest rate in the league behind the Steelers, who are at 13.5 percent. The problem for Pittsburgh is they also don't get any pressure without blitzing, while the Eagles are 11th with a 27.4 percent pressure rate. When you compare the Eagles to every other team when they're not blitzing, Philly's pressure rate is sixth best in the NFL.

Only the Texans have allowed a better QBR than the Eagles this year, at a mark of 31.3. They're certainly getting help from the offense, as Doug Pederson's bunch is following the blueprint he set in Kansas City. Carson Wentz's offense hasn't turned over the ball once in three weeks, which is creating long fields for the opposition. The average possession for Philly's opponents has required 77.1 yards for a touchdown, the fourth-longest field in football, and they're just snuffing opposing teams out on the way. Just 11.8 percent of opposing drives have resulted in points against the Eagles this year. Nobody else in the league is below 20 percent. That's not sustainable, but with Fletcher Cox as the linchpin of an excellent front four, the Philly defense is for real.

Ho-hum, the Pats' defense is great again. For all you hear about how Bill Belichick is a bend-but-don't-break defensive coach, that hasn't been true for years and isn't true again in 2016. The Patriots have allowed just six trips to the red zone in three games, the fourth-lowest figure in football. When teams have made it to the red zone, though, they've scored five touchdowns and kicked a field goal, giving New England the second-worst red zone defense in the league on a per-possession basis. A year ago, they were league average on red zone trips. If there is such a thing as a bend-but-don't-break defense, the Patriots aren't it. They don't bend. (And the red zone defense will get better.)

Jamie Collins has morphed into one of the most dynamic defenders in all of football. (AP Photo/Charles Krupa

The Patriots also have done this while missing two key defensive starters. Rob Ninkovich has been ably replaced at end by Chris Long, who has been unblockable at times and looked like the guy who terrorized quarterbacks with the Rams. Jabaal Sheard, meanwhile, is tied for the league lead with five tackles for loss. And while the Patriots miss Dont'a Hightower, who has missed the past two games with a knee injury, Jamie Collins has been single-handedly mowing grass for the Patriots at linebacker. Both should be back in a couple of weeks, which would just make the New England defense even scarier.

The Belichick (and Matt Patricia) defense also has gotten help from its offense's efficiency; the average starting field position per drive for the Pats is just ahead of the Eagles in third at 77.7 yards away from the end zone. Garoppolo and Brissett are yet to throw an interception, and it's not as though Tom Brady is going to throw a ton of them either. Teams have to dink and dunk down the field, because the Patriots aren't allowing many big plays through the passing game; they are giving up a passer rating of just 33.2 on what the NFL defines to be big passes, 16 yards or more in the air downfield. Only the Ravens are better on deep pass defense.

It looks like the Vikings are picking up where they left off from last year, when they finished fifth in points allowed, but as I wrote after the Teddy Bridgewater injury, the Vikings were actually 14th in defense by DVOA in 2015. They were closer to good than great on defense last year.

Um, that's not the case so far in 2016. The operative word is ferocious. Through three games, the Vikings have forced turnovers on 25.7 percent of opposing drives. 25.7 percent! They have nine takeaways in three games after recording a total of nine during the first eight games of the 2015 season. The defense and special teams have scored as many touchdowns (three) as the offense. Those sorts of rates aren't sustainable -- the Panthers had a league-high 19.4 percent takeaway rate last year, and nobody else was above 16.7 percent -- but the Vikings can still create havoc and remain along the league leaders in forcing turnovers. They also have forced more off-target passes than anybody besides Arizona, at 27.1 percent.

They can do that because they might have the deepest pass rush in football, and that is sustainable. The Vikings lead the league in sack rate (11.3 percent) and are fourth in pressure percentage (31.6 percent). They beat up Cam Newton on Sunday, sacking him eight times on 43 dropbacks while picking the reigning MVP off three times. No. 1 cornerback Xavier Rhodes, making his season debut, shut down Kelvin Benjamin and held him catchless with just one target. Mike Zimmer's group is so fast and rangy that opponents can't really count on their defenders to have a defined role -- just about anybody in the lineup is a threat to blitz on any given play. Bradford may be getting the credit (and admittedly deserved it in Week 2), but this defense is what's carrying the Vikings right now.

No DeMarcus Ware? No problem. The Broncos are slowly preparing for life without the future Hall of Famer on one side of the field, but they have to feel better after seeing what Shane Ray did in his absence Sunday. The 2015 first-rounder picked up three sacks in his first career start, including back-to-back sacks to knock the Bengals out of field-goal range. And of course, while he was doubled all day, Von Miller lurks on the other side of the field with five sacks and six quarterback knockdowns in three games. The Broncos will be just fine.

Denver already has adjusted comfortably to losing Malik Jackson and Danny Trevathan, among others. While Siemian hasn't avoided turnovers in a Wentzian way, the Broncos have been better with the football and managed to make life easier for their defense. Last year's unit faced a league-high 195 meaningful drives and went up against its fourth-worst average starting field position. They've seen just 31 meaningful drives on defense over first three games (on pace for 165), and their average starting field position is almost exactly league average. That's how a great defense can look even better.

Right now, though, I think the best defense in football resides in the Pacific Northwest. The Seahawks led the league in scoring defense and were fourth in DVOA last year, and while we're not through with Week 3 yet, Seattle was tops in DVOA through two weeks. They allowed 18 points to the 49ers, but even that overstates things: Seattle was up 37-3 before the 49ers picked up two rushing touchdowns in the fourth quarter as some sort of Carlos Hyde fantasy fan service.

The Seahawks already are playing lights-out defense. They're second in points allowed per drive behind Philadelphia, first in forcing punts, third in forcing three-and-outs, and second in preventing series from ending with a first down or touchdown. They're third in the percentage of preventing runs from producing first downs. They are good at just about everything, and unlike past years, they haven't been getting a ton of help from the offense in setting the defense up with field position or allowing it to stay fresh on the sideline.

In thinking about what's likely to happen with these defenses over the next few months, though, I have to go with the Seahawks because of how they're performing while missing one key element of their game: turnovers. Seattle has just one takeaway through three games, with opposing offenses having recovered all three of the fumbles to hit the deck with Richard Sherman & Co. on the field. We know enough about this defense to know that the takeaways are going to come. And when they do, the Seahawks' defense is going to be even more terrifying for opposing passers.