His total sets the PGA Tour record for lowest score in relation to par on a par-70 course over a 54-hole stretch. His total of 189 ties the tour record for lowest 54-hole total. John Cook first set the mark in 1996 at the St. Jude Classic and was later matched by Mark Calcavecchia at the 2001 Phoenix Open.
'It's a nice accomplishment to have. I'm glad my name is on there,' said Armour of the scoring record. 'I never shot 21-under in a tournament before.'
Duffy Waldorf, a two-time winner of this event, carded an 8-under 62 to move to 15-under-par 195. Paul Goydos, Glen Hnatiuk, Dan Forsman and Bob Tway share third place at 13-under-par 197.
Armour, who has yet to record a bogey in the tournament, opened with a three- foot birdie at the second on the Resort Course at LaCantera Golf Club. He later dropped a 6-iron within five feet of the cup for birdie at the seventh.
He then rolled in a 25-foot birdie putt at the next hole to make it two straight. Armour, who is looking for his first win since the 1990 Phoenix Open, stuck a pitching-wedge within one-foot of the cup for birdie at the 12th.
After landing his drive in the greenside bunker at the short par-4 16th, Armour got up-and-down for birdie. He sank a 21-footer at the next hole to make it two in a row. He closed in fine fashion as he made a huge putt from over 60-feet out to close with his seventh birdie of the round.
'I'm going to go out and play the first hole and get after it,' Armour said of his plans for the final round. 'No strategy. The weather is supposed to be good tomorrow. You can shoot low on this course. You have to keep going.'
Waldorf started quickly as he sank a 12-foot birdie putt on the first. He sank a 20-footer on the par-3 sixth and made it two straight when he made a five-footer at the next hole.
He dropped a pitching-wedge within three feet for birdie at the ninth, then birdied the par-4 12th to begin a run of three straight birdies. The birdies on Nos. 13 and 14 both came from within four feet.
At the last, he knocked his second shot two feet from the cup for his eighth birdie of the round.
'Today it came together nicely,' said Waldorf, who won here in 1995 and 1999. 'I played real steady golf and I was able to hit the ball close today a lot. That was the difference. I made some key putts on the front side and the middle of the back side, but the last three birdies I had were all pretty short putts.'
Waldorf knows the final round will be tough since he trails by six strokes but he plans on playing his own game.
'Armour will probably be tough to beat,' Waldorf said. 'I'm not going to go out there to try to beat him. I'm going to do the things I did today and put up a score. You don't know what the conditions are going to be. You can't go out there thinking about a score, you have to go out and take what the course gives you and what your game has that day.'
Arron Oberholser fired a 4-under 66 for the third straight round. He stands at 12-under-par 198 alongside K.J. Choi, Stewart Cink and Hal Sutton. Charles Howell III, Richard S. Johnson, Kent Jones, Tim Clark, Tom Lehman and defending champion Loren Roberts are one stroke back at minus-11.