The Washington Redskins’ starting offense remained productive Saturday with third-string quarterback Rex Grossman in charge. Fourth-stringer Pat White also had his moments and the Redskins stayed unbeaten in the preseason by coasting to a 30-7 triumph over the Buffalo Bills at FedEx Field.

Grossman produced 10 points in his two drives with the Redskins’ starting offense. He threw a touchdown pass to wide receiver Pierre Garcon, benefited when a potentially ugly interception was dropped and ended up completing 11 of 21 passes for 171 yards. Grossman directed an additional first-half series with the second-string offense and returned to the game for a pair of third-quarter drives, the second of which yielded a field goal set up by a muffed punt by the Bills, and a fourth-quarter series.

“Rex is a vet,” said Redskins Coach Mike Shanahan, adding that he alternated the two quarterbacks to simulate the requirement for a backup to come in cold and produce immediately. “It didn’t surprise me that he played well, the way he did. But it was nice to see Pat come in and play exceptionally well.”

White got the rest of the playing time at quarterback and ran for a second-quarter touchdown. He connected on 7 of 14 passes for 96 yards. White added 26 rushing yards and led the Redskins to a fourth-quarter touchdown on tailback Keiland Williams’s one-yard run.

“I’m just excited to have an opportunity to get on the field and compete,” White said. “Whether it’s one rep or 20 reps, I’m happy to have this opportunity.”

The Redskins amassed 208 rushing yards, 70 of them by tailback Roy Helu and 52 more by Williams, and upped their exhibition-season record to 3-0 entering Thursday’s preseason finale at Tampa Bay.

“I was impressed with our offensive line and our backs,” Shanahan said. “It’s a group effort any time you run the football.”

The Redskins’ starting defense played three series, forcing two punts but also surrendering a touchdown run by Bills tailback C.J. Spiller. Place kicker Kai Forbath had three field goals for the Redskins, who did suffer one significant loss when reserve cornerback and punt returner Richard Crawford suffered what Shanahan called a season-ending knee injury.

Grossman got the start with the Redskins’ top two quarterbacks, Robert Griffin III and Kirk Cousins, both sidelined. Garcon had said during the week he didn’t expect the preseason quarterback juggling to be disruptive to the Redskins’ starting offense. He and Grossman quickly went about trying to make that true.

Grossman had completions of 21 yards to Helu and 31 yards to tight end Fred Davis on the Redskins’ opening offensive drive. Davis’s catch gave the Redskins a first down at the Buffalo 7-yard line. Grossman failed to connect with Garcon on a first-down fade pattern. But he went right back to Garcon on a second-down slant pattern, and Garcon made the catch and got into the end zone for the touchdown.

“It was pretty good,” Grossman said of his performance. “I definitely had some good plays and got into a rhythm. I felt good about a lot of things I did. A couple plays, I wish I’d have had back. . . . I played all right.”

The bad side of Grossman surfaced on the Redskins’ second drive when he made an ill-advised throw off his back foot and the ball went directly to Bills cornerback Stephon Gilmore for what should have been an interception and a touchdown in the other direction. But Gilmore dropped the pass. Grossman took advantage, teaming with wideout Santana Moss for a 45-yard catch and run on the next play. That led to Forbath’s 28-yard field goal.

The starting defense for the Redskins had mixed results. Josh Wilson, starting at cornerback after missing the Redskins’ first two preseason games following offseason shoulder surgery, got off to a rough beginning when he was called for a 34-yard pass-interference penalty on the Bills’ opening offensive play. But the Redskins defense toughened and forced a punt after a third-down sack of Bills quarterback Kevin Kolb by rookie linebacker Brandon Jenkins.

The second series for the Redskins’ starting defense wasn’t as impressive. A 15-yard face- mask penalty against rookie cornerback David Amerson kept Buffalo’s drive going. The Bills cashed in with a two-yard touchdown run by Spiller. The Redskins did regroup, however, and forced a punt on the Bills’ third drive, after which the starting defense exited. Wilson surrendered a long catch but it was negated by an offensive pass-interference penalty called on Buffalo wide receiver T.J. Graham for shoving Wilson.

White took over for Grossman early in the second quarter and the Redskins kept moving on offense. White’s first drive ended with him scoring a 14-yard touchdown on an option-play keeper. He lost the ball as he dove into the end zone, but the officials ruled that White had possession of the ball when he crossed the goal line, and the touchdown stood after a replay review. Forbath added a 53-yard field goal late in the first half to increase the Redskins’ lead to 20-7.