Mario Tiziani, Brian Nosler and Ryan Miller are knotted in fourth place at 9-under-par 204.
Compton held a two-shot lead after the second round at Mayfair Lakes and was firing on all cylinders on the front nine Saturday with four birdies to make the turn at 14 under.
The 24-year-old, who is seeking his third Canadian Tour title, played a completely different game on the inward half, however, with four bogeys to fall back into a tie for first after a round of 71.
Kovacs vaulted out of the gate with birdies on each of his first three holes. He added a birdie at the fifth and ran off back-to-back birdies starting at the eighth to go out in 29.
The 28-year-old collected two birdies and a bogey over his last nine holes for his share of first.
Heffernan trailed Compton by two to start the day and made up that difference with a round of 69.
Robert Hamilton and Bryan Novoa share seventh place at 8-under-par 205. Jason Allred, Matt McQuillan and Rob Labritz were one shot further back at 7-under-par 206.