LOS ANGELES, February 16, 2014 (AFP) - The 34-year-old birdied the first three holes and five of his first six, finishing with eight birdies in his six-under par 65 for a 12-under par total of 201.
McGirt had a two-stroke lead over George McNeill and Charlie Beljan in the $6.7 million event.
McNeill posted a third-round 66 and Beljan, who lost to John Merrick in a playoff here last year, posted a 68 for 203.
"Hit a lot of good shots and starting out, the hole looked like a washtub," said McGirt, adding that he'd just try to maintain his momentum on Sunday.
"It's just 18 more holes and whatever is going to happen is going to happen," he said. "All I can do is go out and play the best I can."
It was a further stroke back to Brian Harman, whose 68 included an eagle at the first, and Jason Allred -- a Monday qualifier who followed up a second-round 64 with a 67 to stay in the hunt at 204.
The lesser-known names on the upper reaches of the leaderboard had a bevy of big names well within striking distance in a group of seven players on eight-under 205.
They included former Masters champions Bubba Watson and Charl Schwartzel, rising US star Jordan Spieth, eight-time US tour winner Dustin Johnson and early season sensation Jimmy Walker, who last Sunday notched his third win in eight starts at Pebble Beach.
Watson had seven birdies without a bogey in his seven-under 64. Schwartzel posted a second straight 68 and Walker and Spieth both signed for 67s. First-round leader Johnson carded a 68, and Cameron Tringale carded a 67 to join the group at eight-under.
Overnight leader Bae Sang-Moon of South Korea got there with a one-over 72.
Although Bae opened with a birdie, he struggled to build any momentum. A third birdie of the day at the par-five 17th got him back to even for his round, but he closed with a bogey.
Watson, seeking his first win since his 2012 Masters triumph, said cutting down on mistakes was the key to low round.
"Made some silly mistakes, three double (bogeys) the first two days," he said. "Today was just steady ... today just made a few putts, got some balls to bounce my way on the greens and here we are."
McGirt had moved two strokes ahead with birdies at 10 and 11, and he rolled in a 10-footer at the 13th to briefly stretch his lead to three.
A three-putt bogey at 15 was a disappointment, he said, but he got a good break at the last when his tee shot hit a cart path and bounced forward. He was able to reach the green in two to close with a par.
"I'll take 65 any day, especially around this place," said McGirt, who missed the cut in his prior two starts at Riviera.
And the player who has toiled on mini-tours and one-day tours, and thought of packing in his competitive career after Q-School disappointments in 2007 and 2008, was looking forward to having a chance to win on Sunday.
He has two runner-up finishes on the PGA Tour, at the 2012 and 2013 Canadian Opens, and said he'll draw on his experiences in those tournaments.
"Hopefully it will help pull me through it," he said.
Leading third-round scores on Saturday in the US PGA Tour's $6.7 million Northern Trust Open (USA unless noted. Par-71):
201 - William McGirt 69-67-65
203 - George McNeill 69-68-66, Charlie Beljan 67-68-68
204 - Jason Allred 73-64-67, Brian Harman 67-69-68
205 - Bubba Watson 70-71-64, Cameron Tringale 68-70-67, Jimmy Walker 67-71-67, Jordan Spieth 72-66-67, Charl Schwartzel (RSA) 69-68-68, Dustin Johnson 66-70-68, Bae Sang-Moon (KOR) 67-66-72
206 - Bill Haas 72-67-67, Charley Hoffman 67-71-68, Brendan Steele 68-71-67, Aaron Baddeley (AUS) 69-65-72
207 - Luke Guthrie 71-69-67, John Senden (AUS) 71-70-66, Lee Westwood (GBR) 69-70-68, Bryce Molder 69-69-69, Matt Every 69-69-69, Jim Furyk 68-68-71, Robert Garrigus 67-67-73
208 - Hideki Matsuyama (JPN) 70-69-69, KJ Choi (KOR) 69-72-67, Harris English 70-69-69, James Hahn 71-72-65, Blake Adams 67-70-71