49ers want pressure from defensive line full of first-round picks

San Francisco 49ers' Solomon Thomas chases Los Angeles Rams' Robert Woods in 3rd quarter during Rams' 41-39 win in NFL game at Levi's Stadium in Santa Clara, Calif., on Thursday, September 21, 2017. San Francisco 49ers' Solomon Thomas chases Los Angeles Rams' Robert Woods in 3rd quarter during Rams' 41-39 win in NFL game at Levi's Stadium in Santa Clara, Calif., on Thursday, September 21, 2017. Photo: Scott Strazzante, The Chronicle Buy photo Photo: Scott Strazzante, The Chronicle Image 1 of / 4 Caption Close 49ers want pressure from defensive line full of first-round picks 1 / 4 Back to Gallery

After playing in their first two games, Jacksonville defensive end Calais Campbell led the NFL with four sacks and Dallas’ Demarcus Lawrence had 3.5.

The 49ers? They have three sacks. As a team. After three games.

Yes, the pass rush has been a problem.

“It wasn’t good enough, you know?” head coach Kyle Shanahan said.

Shanahan offered that Friday, a day after the 49ers didn’t muster a sack in a 41-39 loss to the Rams. The lack of pressure meant quarterback Jared Goff finished with a clean uniform and stunning stats: In his 10th career start, Goff had career highs in completion percentage (78.6) and passer rating (145.8) and matched a career best with three touchdowns.

“I can’t tell you exactly why, but our guys didn’t win” on the line of scrimmage, Shanahan said. “They blocked us pretty well. We didn’t affect the quarterback enough.”

The 49ers’ new regime was striking a different note before the season after it added another first-round pick to the defensive line. Solomon Thomas, the third overall selection, joined DeForest Buckner (No. 7, 2016) and Arik Armstead (No. 17, 2015).

The 49ers have started their rebuild by acknowledging their roster isn’t close to complete, but general manager John Lynch consistently has pointed to that group, headlined by a host of premium picks.

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“I think we’re all excited about the situation, that through additions and from what we already had, at the defensive-line position,” Lynch said in July on the eve of training camp. “We think we could have a unit that can really be a strength of this team in an area where it’s so important.”

In fairness, the 49ers’ run defense, which allowed the most yards in franchise history last season, has improved. The Niners are allowing 3.5 yards a carry after posting a league-worst 4.8 in 2016.

However, before Sunday’s games, only three teams — the Jets, Dolphins and Buccaneers — had fewer sacks than the 49ers, who were one of two teams to play three games. Miami and Tampa Bay had played just once because their season opener was postponed because of Hurricane Irma.

So what’s the problem? Despite their draft investments, the 49ers still don’t have a classic “Leo,” the outside position reserved for their most disruptive pass rusher. That is, Thomas, Buckner and Armstead are all probably most effective as interior rushers. In addition, Elvis Dumervil, 33, who has 99 career sacks, has been used sparingly: He’s averaging just 16 snaps a game.

Armstead was moved outside to occupy the Leo spot, but, at 6-foot-7, he’s taller than the prototype for the position and remains unproven. Armstead has 5.5 sacks in 27 NFL games after collecting four in three seasons at Oregon. In fact, Buckner, who had 10.5 sacks in his final season at Oregon, is the only one of the first-rounders who had more than 8.5 sacks in a season in college.

On Thursday, Thomas was used more on the edge against the Rams and had one tackle in 38 snaps.

“I do think he was on the outside more,” Shanahan said. “I’d like to see him on the inside. But I don’t only want to see him on the inside. I think he’s capable of being at all four spots. That’s what we like about him. He has the quickness and he has the size to do both, so I think the more we can move him around, the better.”

In three games, Thomas has seven tackles, three for losses, and just one quarterback pressure while playing 59 percent of the defensive snaps (128 of 216). Thomas figures to receive more playing time with defensive tackle Tank Carradine out indefinitely with a high ankle sprain sustained Thursday.

“We need him to get better,” Shanahan said. “He will get better. You know, he’s in his third game in the NFL after missing OTAs, too. I think he has done some real good things. You don’t always get those sack numbers. Those come.”

Shanahan, the Falcons’ offensive coordinator the previous two seasons, noted that Atlanta outside linebacker Vic Beasley took time to acclimate to the NFL. The No. 8 pick in 2015, Beasley had five sacks in his first 20 games. He since has 16.5 in his past 14 games and led the NFL in sacks last year.

“I’m very excited with Solomon,” Shanahan said. “I’m very excited that he’s on our team, and I think he’s going to be a hell of a player for us.”

Eric Branch is a San Francisco Chronicle staff writer. Email: ebranch@sfchronicle.com Twitter: @Eric_Branch

49ers’ next game

Who: 49ers (0-3) at Arizona (1-1)

When: 1 p.m. Sunday

TV/Radio: Channel: 2 Channel: 40/810, 107.7

Monday’s game

Who: Dallas (1-1) at Arizona

When: 5:30 p.m.

TV/Radio: ESPN, ESPN2/1050