SANTA CLARA — Third games of the exhibition season often tip off what’s to come. A year ago, safety Eric Reid made his debut in the 49ers starting lineup.

His rookie year was a hit — often of the bone-jarring variety to opponents.

With the 49ers hosting the San Diego Chargers on Sunday in this preseason’s third exhibition, Reid already has established himself as an old soul in the NFL’s fast-changing world.

—‰’E,’ he’s way beyond his years. He’s doing a lot of things vets do,” said Antoine Bethea, who left the Indianapolis Colts after eight seasons to join Reid as the 49ers’ safety tandem this season.

Reid’s great rookie year didn’t have a great ending, and he hasn’t excused his flaws in January’s 23-17 NFC Championship loss at Seattle.

“I usually don’t forget (mistakes) until the same situation comes up and I make it right,” Reid said recently. “There are a couple plays in the Seattle game I wish I could have had back.”

Such humility and accountability speak to Reid’s upstanding character, a common display over the past year since replacing Pro Bowler Dashon Goldson, who left for Tampa Bay. Reid overcame two scary concussions, adjusted his hard-hitting technique and eventually reached the Pro Bowl as the 49ers’ only willing participant.

While Jim Harbaugh says Reid “continues to grow,” the coach has long preached that players’ greatest strides come between years 1 and 2. To Reid, that means refining “every aspect” of his game, and he said last week he’s especially improved his field vision.

He hasn’t turned a blind eye to last season’s NFC final. He delayed watching that film until well after the Seahawks won the Super Bowl.

“I watched in efforts to get better,” Reid said. “I watched the plays I made mistakes on, plays I could have done better at, in order to play better this year.”

One play: Doug Baldwin got behind Reid and safety Donte Whitner for a 51-yard catch to set up the Seahawks’ first points, a second-quarter field goal. Another play: Marshawn Lynch cut past Reid’s last line of defense at the 30-yard line on a 40-yard, game-tying touchdown run.

Reid wishes he was in better position those plays. But there’s one more play that still haunts him — and the 49ers — and it’s one in which he executed his assignment.

Midway through the fourth quarter, Reid tackled Jermaine Kearse at the 1-yard line. Amid the rubble was NaVorro Bowman’s left knee, which buckled upon Reid’s impact with Kearse and resulted in surgery that will sideline Bowman into November.

“That play has been in my mind for a very long time,” Reid said. “It’s made his injury extremely personal for me. I look forward to him coming back, because I was in on that play. I pray for him.”

Reid had his own scary moments last season. He twice left games with a concussion but each time recovered to start the next week. He learned to better size up his opponents, and “didn’t try to kill everybody like I usually try to do.”

“He’s a smart guy,” said defensive coordinator Vic Fangio, “and smart guys improve faster than guys that aren’t smart. He’s much improved over where he was last year.”

Harbaugh likes what he’s seen out of the Reid-Bethea tandem, noting: “It’s coming along really good. Our safety play has been very good, has been in practice and in games.”

A concussion knocked Bethea out of last Sunday’s exhibition loss to the Denver Broncos, and he will sit out the Chargers game.

Bethea is eager to resume what he predicts will be a “good working relationship” with Reid.

“The way he’s verbal, some young guys are not verbal until the third or fourth year when they’re really, really comfortable,” Bethea said.

That comfort level will be tested with a revamped secondary with new opening-day starters at cornerback, projected as Tramaine Brock and Chris Culliver, with rookie Jimmie Ward at nickel back.

Reid’s improved field vision should help immensely.

“I’m seeing more,” Reid said. “Last year when I got here they would say I had a half (of the field), so I was just in a half. Now I’m going through progressions and just trying to see more.”

His sights ultimately are set on a familiar task that hasn’t been fulfilled by the 49ers the past 19 seasons. “Our ultimate goal,” Reid said, “is to win a Super Bowl.”