Jeff Gove came in with a 3-under 69 and is at minus-8. Daniel Chopra and Geoff Ogilvy each posted 71s and are tied for fourth at 7-under-par 209.
Toms bogeyed the second on the Sunrise Course at Country Club of Mirasol to open his round. The former PGA champion went birdie-bogey from the fifth and eighth to turn at minus-9.
The 39-year-old birdied the 10th and was two shots clear of Donald, but things went downhill from there. Toms bogeyed 13 and 14, then 16 and 18 to fall three behind the leaders.
Donald was steady to start his day with five straight pars. He rolled in a birdie on the sixth to get to 6 under. He came back two holes later with another birdie.
The Englishman birdied the par-5 12th to get within two of Toms. Donald's birdie from 12 feet out on the 14th moved him to minus-9.
Donald, whose lone PGA TOUR win was at the 2002 Southern Farm Bureau Classic, slipped back to 7 under when he dropped a shot on 16, as he could not save par from over the green. Donald came right back with a 5-foot birdie putt on the 17th to close at 9 under.
'I think the way the greens are, you need to be very accurate with your irons,' Donald said. 'Whether there's wind or not, the most important thing out here is being able to control the distance of your irons and the direction. I've done that pretty well the last couple of days.'
Donald owns the lead after three rounds for the third time in his career. Twice in 2005 he led after the third round, but finished second both times.
'I've been at the front a couple of times and won back in 2002, but I haven't been in that position too many times over here in the United States, so we'll see how it goes,' Donald said.
Mayfair, who played alongside Toms, could not save par from a greenside bunker on two to open his round. He got that stroke back with a 12-foot birdie putt on four.
The 39-year-old gave another stroke back to par on the sixth, but made a 14-foot birdie putt at the next to get back to 9 under.
Mayfair fell off the pace with consecutive bogeys from the 10th. The 1995 TOUR Championship winner ran home a 25-foot eagle putt at 17, though, to jump back into a tie for the lead.
'When you get to 13, 14, 15 and 16 out there, they are the difficult holes,' said Mayfair, whose last win was at the 1998 Buick Open. 'Fortunately we got to play without much wind today, which the first two days, we played in a lot of wind.
'You get through those holes, and you know you have 17 ahead of you, which is a good eagle or birdie chance, and 18 is a tricky little hole, it doesn't look like much, but it can grab you real quick.'
Mathew Goggin had the round of the day as he fired a 7-under 65. That jumped him from a tie for 57th to a tie for eighth at 5-under-par 211. He was joined there by Frank Lickliter and Pat Perez.
Padraig Harrington, the defending champion, had two birdies and two double bogeys en route to a 74. That dropped him into a tie for 11th with Dudley Hart and Tom Pernice, Jr. at minus-4.