Edwards, the second-round leader, had an even-par 72, while McNulty shot a 2-under 70. They finished 54 holes at 12-under-par 204.
D.A. Weibring, who won the 3M Championship two weeks ago, shot a 4-under 68 and is in third place.
"You play yourself into position and then hope things go your way," said Edwards. "You can't be on too much of an emotional roller-coaster out here."
Neither co-leader has won a major championship on the Champions Tour.
McNulty owns five wins -- all coming in 2004 and 2005, his first two years on tour. Edwards' only tour victory came last year at the 3M Championship.