The Senior PGA winner in late May, Lehman overcame a double-bogey 6 on No. 8 with three birdies on the back nine in cool, overcast conditions. He was 11 under.
"There is potential for a low round out here. The opportunity will be out there for guys to go 5-, 6-, 7-under par," Lehman said. "A two-stroke lead is nice, but there's really no room for error."
Bernhard Langer, attempting to become the second player in Champions Tour history to win three consecutive majors, shot a 69 to join Fred Funk (70) and J.L. Lewis (66) at 9 under in the fourth of the tour's five majors.
"Langer has had such a great year the last month," Lehman said. "It's going to force me to have to play well, but he's only one of so many."
Two strokes behind D.A. Weibring after two rounds, Lehman took the outright lead with a birdie at 14, then made a 45-foot birdie putt at 17 to increase his advantage.
Lehman, who opened with rounds of 67 and 69, said the round "tee to green, was my most solid round of the week."
Langer, the Senior British Open and U.S. Senior Open winner in consecutive weeks, made a 15-foot eagle putt on the par-5 12th en route to his third straight 69. Gary Player won three consecutive senior majors, the final two of 1987 and first of 1988.
Langer also could join Jack Nicklaus as the only player to win three senior majors in a season. In 1991, Nicklaus won the PGA Seniors, U.S. Senior and Tradition.
The German star is in line for a possible berth on the European Ryder Cup team as a pick by captain Colin Montgomerie. Langer refused to look ahead.
"I try not to go there," he said. "It doesn't do any good, anyway. It's Saturday. Why wonder what's going to happen tomorrow or the next day?"
Lewis, who turned 50 in July, had a bogey-free round. He'll play Sunday in the final group with Lehman and Langer.
"I feel like I can be very competitive, but you still have to play good. I hope I'm there. That would be fun," Lewis said.
Funk continued his strong play in the Tradition. He won the 2008 event finished fifth last year.
Weibring (74) and Chien Soon Lu (65) were 8 under along with John Cook (68), Bob Gilder (69), Bob Tway (71) and Gil Morgan (71).
Weibring made four bogeys on the back nine, though he finished with birdies at 17 and 18. Lu, who didn't play competitive golf from 2001-08 because of back problem, had the best round of the day. He birdied five of his first six holes.
Gilder, the lone Oregonian in the field, briefly tied for the lead early on the back nine, but made a bogey at 18.
Cook lost in a playoff to Mike Reid in the 2009 Tradition.