Langer, who shot 68 in the first round and 67 in the second, had five birdies Saturday at the Royal Birkdale Golf Club. He bogeyed the par-3 14th hole but got back on track quickly with birdies on 15 and 17.
He finished the day at 9-under 201.
Frost shot a 68 Saturday despite three bogeys on the back nine to take a 6-under 204 into the final round.
Mark Wiebe, who shared the lead with Langer after firing a 65 Friday, shot an even-par 7 Saturday to finish the round in third place, four shots behind Langer.