SANTA CLARA — When you think of the Cover-3 defense, you should think of Earl Thomas, Kam Chancellor, and Richard Sherman.

After all, the former Seahawks trio — known as the Legion of Boom — epitomized and popularized the scheme and was the backbone of an all-time great defense that lifted Seattle to its first and only Super Bowl title in 2013.

I apologize for canonizing the 49ers’ rivals, but the context is important: The Cover-3 made life so difficult for 49ers head coach Kyle Shanahan when he was an offensive coordinator for Washington, Cleveland, and Atlanta, he decided to adopt it his team’s defensive scheme when he took over in San Francisco.

Year One of the Cover-3 in did not go well for the Niners, who finished 26th in the NFL in defensive DVOA last season and 28th in pass defense (allowing opponents to perform 20 percent better than average per the air), so Shanahan and general manager John Lynch went out and signed Sherman to be the team’s No. 1 cornerback.

But the 49ers opted to stand pat at the safety positions heading into 2018.

No one can match what Thomas and Chancellor did for the Seahawks, but the 49ers feel they have solid, young facsimiles of those prototypes already in place in Adrian Colbert and Jaquaski Tartt.

They’re not at that elite, LOB-level yet — let’s not blaspheme — but defensive coordinator Robert Saleh (who coached in Seattle from 2011 to 2013) told me getting to that level is a possibility for the young safeties.

And watching the duo work during Training Camp, it’s easy to believe him.

These guys might not be well-known around the league, but they’re shaping up to be one hell of a safety duo — and that could mean big things for the 49ers in 2018 and beyond.

You might recall that Colbert and the 49ers thrived once the 2017 seventh-round pick out of Miami and Texas was moved into the starting free safety role in Week 10. In fact, the Niners went 6-0 in games Colbert started last season — Jimmy Garoppolo was only 5-0.

The free safety in a Cover-3 scheme is a position that isn’t far behind the quarterback in terms of importance — hard to win if you don’t have baseline competent play at the position. Look at what happened to the Seahawks in 2016, when Thomas broke his leg in Week 12: with Thomas on the field, the Seahawks had the fifth-ranked pass defense (per Football Outsiders) in the NFL — without him, they ranked 30th.

Yes, the single-high free safety is the lynchpin of the Cover-3 system — tasked with patrolling the middle of the field, solo, and charged with cleaning up any messes from sideline to sideline, offensive backfield to the end zone.

It’s an extremely difficult job, but after a strong debut, Colbert is continuing to prove he’s up-to-the-task.

“He has tremendous range, to go sideline-to-sideline, but also tremendous tracking ability,” Saleh said of Colbert. “He eliminates all explosive [plays]. We call him The Eraser — when something slips in the front, he has to stop it from being a game-changing play. He has tremendous instinct in that regard… and he loves contact.”

Sherman told me that the Cover-3 system is one that takes a few years to learn. It’s not complicated — every man has one job — but it’s exacting.

Colbert agreed with that assessment and said that the difference between his first NFL Training Camp and second has been huge, in no small part because of a special offseason workout.

Colbert idolizes Thomas, as any good free safety should. He has all of Thomas’ film since 2014 and, thanks to their University of Texas connections, he was able to work out with him in Austin for three days in April.

“I had other plans prior to that, but I canceled those — it’s ET,” Colbert said. “I see a lot more now that he gave me a lot of tips on my game… It’s awesome. I’ve looked up to the guy forever and just to sit down and talk about [things] and get on the field and do work with him is crazy.”

And Sherman is seeing the fruits of that labor:

“His breaks have gotten better,” Sherman, a foremost expert on the Cover-3 defense, said of Colbert. “He did kind of rounded breaks — he’d waste steps — but now he’s putting it together. Great eyes, great discipline, great range.”

Colbert wasn’t the 49ers’ first choice at free safety last season — he was fourth on the depth chart to start the year. It seems laughable now, but remember, he was a seventh-round pick who only started six games in college. This guy came out of nowhere.

In fact, it was Tartt who was the 49ers’ Week One starter at free safety. In a crowded (and oft-injured) secondary alongside Jimmie Ward and Eric Reid, Tartt bounced back and forth between free and strong safety last year before breaking his arm in Week Nine (creating an opening for Colbert).

Tartt’s versatility has made him a valuable player for the 49ers over the last three seasons and netted him a two-year contract extension this past April, but heading into 2018, he’s locked into a strong safety role that puts a premium on both that versatility and his prodigious thump.

Few people knew about the college Tartt attended, Samford (it’s in Alabama), but his reputation as a big hitter was no secret when he was selected in the second round of the 2015 NFL Draft.

That said, the move to strong safety wasn’t Tartt’s idea. He liked playing free safety. As an immensely confident person (his Twitter feed can attest to that), he enjoyed the responsibility of being the all-important single-high guy.

But he wasn’t peeved when he was moved to strong safety in favor of Colbert this past offseason.

“You’ve seen AC’s tape — he deserved that spot,” Tartt said. “Him and his range — his range hasn’t got to my level yet, but it will get there.”

In the meantime, Tartt will move to a position he was, like it or not, tailor-made to play: The strong safety in the Cover-3 scheme is a gap-stuffing linebacker on run plays and a full-scale defensive back on passing downs. There aren’t many guys who can do both jobs at a high level — even Chancellor had his problems in coverage — but Tartt has the full repertoire necessary to thrive.

“He’s very instinctual, athletic, and powerful in the box,” Saleh told me. “For him, the reality is that he could play free safety too, he just has a little bit different of a body type.”

Football is an immensely complicated game, but the formula for the 49ers in 2018 is relatively simple: If the Niners can keep Jimmy Garoppolo upright and play even league-average defense, they have a chance to be much better than just average.

And without much of a pass rush, strong play in the defensive secondary will is absolutely vital to reaching that level. But with Colbert and Tartt in their new, full-time roles, it’s hard not to imagine something far more impressive than baseline competence coming this season.

Because the two safety spots are arguably the most important positions in a Cover-3 defense, and the 49ers look like they have the guys for the job.