Taiwan's C.T. Pan (67) and American Billy Horschel (67) were both a further shot back after each made a bogey at the last to drop into a share of fourth place.
Poulter, whose Houston win this month earned him a berth in last week's U.S. Masters, is showing no signs of fatigue despite playing in his sixth PGA Tour event in as many weeks.
"Still standing, that's a good thing," said Poulter, who entered the final round at Harbour Town last year three shots off the lead before a 73 left him in a share of 11th place.
"Six weeks, it's unusual for me to play that many weeks in a row but I can muster up another day... see if we can pull it out of the bag again."
Overnight leader Bryson DeChambeau (75) faltered early with a triple-bogey at the par-five second and added two double-bogeys over his final six holes to sit seven shots back.
World number one Dustin Johnson (72) was in a distant share of 41st place and 10 shots behind Poulter after mixing four bogeys with three birdies.
Due to the possibility of strong afternoon thunderstorms on Sunday, the final round will feature threesomes going off split tees starting at 7 a.m. ET (1100 GMT).
(Reporting by Frank Pingue in Toronto; Editing by Clare Fallon)
