TORONTO, 25 July 2015 (AFP) - Hearn had five birdies and a bogey in a four-under par 68 on Saturday, his 54-hole total of 201 putting him two shots in front of two-time Masters champion Watson and Australia's world number nine Day.
Hearn led by as many as four shots on the back nine, but Watson closed the gap with a 68 that included five birdies and an eagle on the par-five 13th while Day roared back from a double-bogey six at the 11th with birdies at six of his last seven holes.
That included a run of four birdies to end the round for the Australian, who is coming off a tie for fourth at the weather-disrupted British Open that ended at St. Andrews on Monday.
If Hearn can win his first US PGA Tour title on Sunday, he'll become the first Canadian since Pat Fletcher in 1954 to lift the trophy.
It's a feat Canadian fans are desperate to see.
"The ovation on the first tee was something I will remember for a long time," Hearn said of his reception on Saturday, when he started the round four strokes behind overnight leader Chad Campbell.
"It was a lot of fun out there, a really special day."
Hearn got off to a solid start with birdies at the second, seventh and eighth holes and stretched his lead to four shots with back-to-back birdies at 15 and 16.
However he gave a stroke back with a bogey at 17 and settled for a par at the par-five 18th.
"It's a dream to be in the final group tomorrow and I like the way I'm playing," Hearn said. "It's been so long since a Canadian won and everyone wants to win their national championship, but it's hard to win any tournament on the PGA Tour.
"If I could win tomorrow, it would be the highlight of my career."
However, he has proven winners nipping at his heels.
Watson, who finished the second round with two eagles on his last three holes, rolled in an 18-foot eagle putt at 13 and birdied the last.
"I knew I had to make that putt on the last hole to get into the last group, so that was really big," said Watson, keen to bounce back after missing the cut at the British Open. "I hung in there and made some big putts."
Brooks Koepka and Michael Putnam shared fourth place on 12-under 204, both Americans carding 68s.
Campbell followed up his second-round 63 with a 75 and was among a group on 205 that also included Colombian Camilo Villegas, US veteran Jim Furyk, Argentina's Emiliano Grillo and American Johnson Wagner.
Leading third-round scores on Saturday in the USPGA Tour's Canadian Open (USA unless noted, par-72, a-amateur):
201 - David Hearn (CAN) 69-64-68
203 - Bubba Watson 68-67-68, Jason Day (AUS) 68-66-69
204 - Michael Putnam 71-65-68, Brooks Koepka 68-68-68
205 - Camilo Villegas (COL) 69-69-67, Jim Furyk 68-69-68, Emiliano Grillo (ARG) 64-72-69, Johnson Wagner 67-66-72, Chad Campbell 67-63-75
206 - Ollie Schniederjans 71-69-66, Ricky Barnes 68-70-68, Charley Hoffman 71-66-69, Alex Prugh 67-70-69, Erik Compton 66-69-71
207 - Jonas Blixt (SWE) 69-69-69, Matt Kuchar 69-69-69, Pat Perez 69-69-69, Austin Cook 68-70-69
208 - Roberto Castro 69-72-67, Adam Hadwin (CAN) 74-67-67, Hudson Swafford 71-68-69, Cameron Percy (AUS) 72-67-69, James Hahn 71-68-69