The Texan ' small in stature, but heavy in grit ' stands at 16-under-par 197, three shots clear of Cameron Beckman (67). The mark is a new 54-hole tournament record by a stroke.
'Fifty-four-hole records are great,' Leonard said. 'But we want to have 72-hole records. Those are what counts.'
Mickelson (72) and Heath Slocum (66) are four back. Love (72) and Angel Cabrera (70) are tied for fifth place, at 10-under.
Leonard started the day two shots back of overnight leader Mickelson, but fell four off the pace as he bogeyed the first and Lefty birdied the second.
That proved, however, to be one of only two dropped shots on the day for Leonard. He birdied Nos. 4, 5, 6 and 8 to tie Mickelson by turn, at 13-under. Mickelson followed his birdie at the second with a bogey at the third, and then parred his remaining six holes going out.
Love, meanwhile, turned in 1-under 35 to get within one, but missed a three-foot par putt at the 10th to fall two back. The back nine was a series of alternating jumps on the up- and down-escalators.
The four-time Heritage champion sank a 15-foot birdie at the 12th; bogeyed the 13th, when his approach shot buried next to the wooden planks in the greenside bunker; missed another three-footer for par at No. 14; made a 12-foot birdie putt on 15; and three-putted the final hole to finish the day six off the lead.
Mickelson was only slightly more consistent on the back.
He hit the pin with his approach shot at the 10th, but lipped out the 10-footer for birdie. He stuck a wedge to six feet, and converted the putt at No. 12; however, he struggled with his short irons down the stretch.
Mickelson buried his approach shot, from 129 yards, into the front greenside bunker at the par-4 13th and made bogey to fall one back of Leonard. He made it back-to-back bogeys when he missed the green left and long at the par-3 14th.
Only 90 yards from the hole on the par-5 15th, Mickelson blasted his third shot over the green, and was forced to make a 12-foot par save ' the first of four successive pars coming in.
'I didn't play as well as I'd like. I had trouble making birdies,' Mickelson said. 'I felt like I did a good job keeping it around par because I did not hit very many greens today.'
Leonard, seeking his first victory since his successful defense in last years Texas Open, showed a far steadier hand.
He stuffed a 9-iron, from 149 yards, to a foot on the par-4 12th. The 1997 British Open champion then made a 15-foot birdie at No. 15, and punctuated his round with an approach to two feet on 18.
While the big names stole the spotlight Saturday, a couple of lesser knowns snuck up the leaderboard, and into contention.
Beckman, who won last years final full-field event, the Southern Farm Bureau Classic, earned his way into the final twosome with a back-nine 32. Slocum, a three-time winner on the 2001 Buy.Com Tour, didnt drop a shot in a five-birdie third round.
Full-field scores from the WorldCom Classic