Harrington turned in a four-under-par 68 in the third round to move to 13-under for the tournament. He owns a one-stroke lead over Peter Hansen, Miguel Angel Jimenez and Shiv Kapur. Nine players are within three strokes of the lead going into Sunday's final round.
Harrington started fast, recording birdies on Nos. 1 and 2 and again at the fourth hole. He bogeyed No. 8. Harrington's birdie at No. 12 pulled him into a tie for the lead and at one time four players were atop the leader board.
But Harrington kept steady, playing par golf until the 18th when he made an 8-foot putt for his 68 and the outright lead.
"I've done particularly well for three rounds and haven't seen the scoreboard until the 18th green each day, so I certainly won't be looking at one individual out there," Harrington said.
"I will be looking at only one individual and that will be me. I won't worry about anyone else."