Scott Verplank moved from a share of 11th place to solo third with a 7-under 65. He completed 54 holes at 16-under-par 200. Cameron Beckman is one stroke further back at minus-15 after a 68.
Vijay Singh, the defending champion and World No. 1, is one of four players at 14-under-par 202 after a third-round 65.
Lehman struggled early as he dropped a stroke at the par-4 second. The 1996 British Open champion erased that mistake with an eagle at the par-5 fourth.
The 45-year-old Lehman drained a 10-foot birdie putt at the seventh and came back with a 7-footer for birdie at eight. The second birdie gave him the outright lead at minus-15.
Lehman two-putted for birdie at 10 to remain out in front. The five-time winner on the PGA Tour then holed a sand-wedge from 90 yards out for eagle at the par-4 13th on the Magnolia Course.
The eagle jumped Lehman to minus-18. He three-putted for bogey at the 16th to fall into a share of the lead with Baird. Lehman missed birdie putts on the last two holes to remain at 17 under par.
'Well, it was a good day. Briny played extremely well on the back nine and I played extremely well on the front nine,' Lehman said. 'Whoever makes the most birdies is the way it works. They put the pins in really difficult spots and sometimes it gets more difficult to make birdies, but there's not a whole lot of rough. You can definitely make some scores out there. I think no matter what they do with the pins, it's still going to be a birdie-fest.'
Lehman has held at least a piece of the 54-hole lead the last two weeks, but was unable to visit the winner's circle on Sunday in either Las Vegas or Greensboro.
'I feel like I had a great game plan over the last two weeks. I feel like I was very calm on the golf course and very focused and played well and just made a couple too many mistakes,' said Lehman. 'Tomorrow I'm going to need to go low here.'
Baird opened with seven straight pars after beginning the round one stroke clear of the field at 13 under. He dropped a stroke at the eighth, but came right back to birdie No. 9.
The 32-year-old Baird made it two in a row as he birdied the 10th and capped a run of three straight at the 11th to move to 15 under. Baird climbed within two strokes of Lehman's lead with a birdie on the par-5 14th.
Baird dropped his second shot within 2 feet of the hole at the 16th. He kicked that putt in for birdie to join Lehman, who bogeyed the same hole, in the lead at minus-17. Baird two-putted for par on the last two holes to remain atop the leaderboard.
Singh was hot early in his round. He birdied the second, but faltered to a bogey at the next. The Fijian ran off four straight birdies from the fourth to jump to minus-11.
Around the turn, Singh birdied three of four holes from the 10th to briefly get a share of the lead at 14 under. He missed several makeable putts coming in as he parred his final five holes to end three shots off the pace.
'I drove the ball beautifully and hit a lot of good iron shots,' Singh said. 'The ones that I needed didn't go in coming in. I got myself back in there. Hopefully I'm not too far back at the end of the day.'
Singh is joined in a tie for fifth at minus-14 by Geoff Ogilvy, Tim Clark and Mark Calcavecchia.