After a 15 foot eagle putt on the long first had taken Harrington into a share of second spot with Donald, two behind American Dustin Johnson, he was twice in the lake at the 438 yard third.
Even with a 14 foot putt at the end of it a triple bogey seven went on the Dubliner's card and, with Johnson two-putting the opener for birdie, it was asking an awful lot to come back from that.
At least Donald limited the damage to a bogey after following Harrington into the water with his approach from the rough to the third.
Chasing his second WGC title in three weeks - and one which would take him to second in the Official World Golf Ranking ahead of Lee Westwood - the 33 year old then only just carried the lake at the short fourth.
But a neat up and down saved Donald's par there and on 11 under and three behind his hopes remained high.
Johnson played the first four in one under, but was caught when Hunter Mahan, the leader after the first and second rounds, had four birdies in the first six and Matt Kuchar had three in five.
On 14 under they were two shots ahead of their compatriot Nick Watney.
Donald was in a tie for fifth with 21 year old Rory McIlroy, the Northern Irishman two-putting the first for birdie and then salvaging a par on the fifth by chipping in after going from fairway bunker to greenside trap.
Scotland's Martin Laird resumed on nine under, but after making birdie from five feet at the second he bogeyed the sixth before making birdie like Harrington on the seventh. It put both of them on nine under.
Only one behind them in 13th spot was Tiger Woods, who from 30th at the start of the day was at last finding form - too late for this week, but encouraging with The Masters Tournament only three weeks ago.
Woods turned in 34, then birdied the 11th, 12th, 16th and 17th, where his approach ran over the edge of the hole to three feet.
“I felt that as the week progressed this week, I felt like I hit a lot of good shots but then I would get sidetracked there for a little bit, and didn't really know what the fix was a couple of times,” said Woods after signing for a 66.
“Today, I hit a lot of good golf shots and when I did mis hit one, I knew what the fix was right away, boom and I got right back on my run of hitting good shots again. That feels good.”
Westwood gave himself a boost too with four birdies in the first eight, but three-putted the ninth and with one to go was five under.
The pressure seemed to be telling as Kuchar missed a six foot chance on the sixth and bogeyed the next, then Johnson missed a five foot birdie chance at the seventh and Mahan bogeyed the ninth.
Johnson was back in front on his own on 14 under, with Kuchar and Mahan 13 under and Watney, Donald and Italian Francesco Molinari 12 under, while McIlroy bogeyed the seventh and had four shots to make up.
Johnson parred eight in a row after his birdie start and it cost him the lead when Watney, runner-up to Phil Mickelson two years ago, had three in a row from the eighth to reach 15 under.
Kuchar also birdied the tenth and joined Johnson on 14 under, while Donald's pitch to five feet on the long eighth moved him alongside Mahan at 13 under.
Watney's three-putt bogey at the 11th brought him back alongside Johnson, while Kuchar would have been alongside them but for doing the same. Mahan's chances nosedived when he bogeyed the long 12th and short 13th.
Moving in the opposite direction, though, was Dane Anders Hansen, whose four birdies in six holes at the start of the back nine brought him only one behind.