With a score of five-under 66 Piercy continued the momentum he created earlier on, having ended the previous day on a three shot lead.
There was a six and a half hour delay to Saturday's round due to inclement weather, but Piercy sits at 18-under 195 heading into Sunday's final round.
That was all done while dealing with a blister. Piercy walked between shots with his right shoe in his hand, only returning it to his foot between shots. Although kept the shoe on when putting.
Argentina's Emiliano Grillo was steady trailing for the second day in a row, on four-under 67.
Behind him were Americans Doug Ghim and Tony Finau.
Piercy birdied seven holes and bogeyed two on the third day in Blaine, while Grillo pulled off an eagle on top of four birdies and two bogeys.
Ghim and Finau both earnt themselves six birdies.
"It feels great," Piercy said. "I obviously would have liked to have finished off the last hole a little bit better.
"My goal, once I got off to kind of a good start, was to get to 21 and keep the pedal to the metal and just try to go. The last four, five holes, (there were) kind of some tired swings and a little bit of mental grind today. So I'm happy that I actually gained a stroke from three to four and let's go do it again on Sunday."
Piercy will be paired with Grillo in the final round, and believes catching Piercy will be a big task.
"I'm pleased the way I closed the round," Grillo said.
"Piercy played phenomenal today.
"If he brings half of his game from today to tomorrow, that's going to be very tough. Obviously you're looking at a very low round tomorrow to have a chance.
"But I'm just going to go out and try to have fun and, like I say, try to play my best."