At 12-under-par, Cotner's nearest competitor is Jody Bellflower, who shot a third-round 69 in Junction City, Ore. Ted Purdy and second-round leader Todd Barranger are two shots off the lead at 10-under-par.
Cotner, currently 94th on the money list, carded seven birdies and no bogeys on Saturday. He needed only 22 putts to make his way around the Shadow Hills Country Club.
Barranger went out in 2-under-par 34 in the third round, but posted three bogeys in a five-hole stretch on the back nine. A birdie at the par-5 18th gave Barranger an even-par round of 72.
Rob Bradley, who started the day just one off the lead, dropped into a tie for 18th at 4-under-par. Bradley is now eight shots back of Cotner after a no-birdie, five-bogey 77.