Ridings (71), winless in 154 starts on the PGA Tour, finished with a 1-under 71 to match Howell (69) at 8-under 208.
Jeff Overton (73), who led both the first and second rounds by one shot, was tied for third at 7 under with Brian Davis (69), Jason Day (71) and Kyle Thompson (69).
Another shot behind were Dustin Johnson (70) and Robert Allenby (71), while Davis Love III (66), Charles Warren (67), and Mark Hensby (72) were tied at 5 under.
Because of two weather delays on Friday, 23 players failed to finish the second round and had to complete it Saturday morning. Love, coming back from an ankle injury suffered a year ago, then reeled off four straight birdies on the back nine to start the third round and move into contention, an early indication the 7,482-yard Atunyote Golf Club course might be playing easier because the weather wasnt quite so harsh. The players also were allowed to lift and clean their balls.
Ridings trailed Overton by one shot at the start of play and played the front nine to even par with a pair of birdies and two bogeys. He sank a 23-foot birdie putt to reach 8 under at the par-3 11th hole and gained a two-shot lead with a 16-foot birdie putt at No. 15 as the sun broke through and the temperature made it into the mid-50s.
Ridings, who has missed 12 cuts this year, including six in a row, had sole possession of the lead until he mishit a 6-foot putt at No. 18.
Overton never mounted a charge. He bogeyed No. 1, reeled off 13 straight pars, and made bogey at 15 to fall to 6 under as he sputtered with his putter for the second straight day. Overton, who needed 34 putts on Friday, had 32 on Saturday but rallied with birdie at 16, his only birdie of the day.
Hensby, who began the day three shots behind, made three birdies on the front side and another at the par-5 12th hole to take sole possession of the lead at 9 under, one shot ahead of Ridings and Thompson.
Hensbys lead didnt last long. He bogeyed Nos. 13 and 14, just as Thompson had done only moments earlier. Thompson sank a 58-foot birdie putt at No. 15 to join a logjam of six players at 7 under.
You could take all the putts I made Thursday and Friday and they wouldnt add up to that one, said Thompson, who finished with his third straight birdie at 18.
The first two days were plagued by temperatures in the 40s, 20 mph wind, rain, and even a brief shower of hail that caused one of two delays Friday.
The fickle conditions continued Saturday. Although there was just a slight breeze and the sun was in and out of the overhanging clouds, a misting rain again began falling while the lead threesome was playing the front nine and the temperature hovered around 50, forcing the players to again wear layers and fight to keep warm and loose.
It was not nice. I can tell you the holes it was warm on, said Love, who hadnt played a tournament since mid-August and whose best finish this year is a tie for 19th at the British Open. Ive got to get my confidence back, get into contention and win a few tournaments.
Love opened the tournament with a 3-over 75 that included 37 putts. Aside from a 15-footer and a 6-footer, the longest putt he made on the day was 3 feet and his total distance of made putts added up to only 47 feet.
It was much different Saturday. Love, who began the day nine shots off the lead and started at the 10th hole, made a strong charge from the start. He hit to 5 feet and made birdie at the par-4 10th hole, rolled in a 27-foot birdie putt at the par-3 11th hole, hit to 7 feet and made birdie at the par-5 12th, and finished the rush by hitting inside 3 feet at the par-4 13th hole.
That had Love at 5 under for the day at the turn, and Overtons group had yet to tee off. Love reached 6 under at No. 4, hitting out of the deep left rough from 125 yards to inside 11 feet and curling in another birdie putt, and carded his sixth birdie at the par-5 fifth hole by two-putting from 92 feet.
Loves luck ran out when he drove into the primary rough at the par-4 seventh hole and three-putted for bogey from 40 feet.