His aggregate of 188 tied the record low for 54 holes on the PGA Tour, set by Steve Stricker at the 2010 John Deere Classic.
"Just patience," the long-hitting 23-year-old told Golf Channel when asked what had been the key to his bogey-free round. "I didn't hit my iron shots as well I felt like today as I did the first two days.
"I had a lot of scoring opportunities and scoring clubs, I just hit really poor iron shots to 25-30 feet. But it was great. I managed my day really well and to shoot five-under not having my best stuff was great. And going bogey-free was awesome."
Thomas, who opened with a magical 11-under 59 at Waialae to take control of the tournament, is seeking his third victory of the young season, and a second in a row, and will head into the final round seven clear of fellow American Zach Johnson (65).
England's Olympic champion Justin Rose (66) is a further stroke back at 14-under, level with Americans Gary Woodland (68) and Hudson Swafford (66).
American journeyman Kevin Kisner, who made the cut right on the number, narrowly missed a 10-foot eagle putt on his final hole that would have earned him a 59 and settled for a 60 that left him nine shots off the pace.
Thomas, five strokes clear overnight after setting a PGA Tour record low for 36 holes with a 17-under total of 123, had to wait until the par-five ninth before he picked up his first shot of the day with a comfortable two-putt from range.
He then birdied the driveable par-four 10th, getting up and down from a greenside bunker, and the par-four 14th, where he hit a superb second shot from the right rough to within three feet of the cup, to extend his lead to six strokes.
Thomas also birdied the 16th, draining a 10-footer, and the par-five last, where he missed the fairway off the tee and clipped palm trees fronds with his second shot before hitting a wedge to 14 feet and sinking the putt.
"I just have to go out and do what I did today," Thomas said of Sunday's final round. "I played with a big lead today and felt like I handled it pretty well."
(Reporting by Mark Lamport-Stokes in St. Augustine, Florida; Editing by Peter Rutherford/John O'Brien)
