Rob Oppenheim carded a 2-under 70 and is alone in fourth place at 7-under-par 209.
Compton struggled on the front nine with a birdie and a bogey but a double bogey at the par-4 ninth cost him his lead. He rebounded with back-to-back birdies at 10 and 11 to match Martin in first.
Compton traded a bogey and a birdie over the 14th and 15th but the third-round lead came down to the par-5 18th.
Martin, who remained in a tie for the lead with Compton at the closing hole, made double bogey while Compton tallied a birdie, giving the Florida native his three-shot advantage.
Compton will be in search of his second win on the Canadian Tour. He captured last year's Guadalajara Classic.
Martin was 2 under on his round until the fateful turn at 18. He shot an even-par 72 to reach minus-8.
Gallie collected six birdies and no bogeys through his first 15 holes but trouble loomed. He bogeyed 16 and 17 to shoot a 68 and join Martin in a share of second.
Alex Rocha (67), Jeff Wood (68), Dirk Ayers (68), Rob Johnson (71) and Iain Steel (64) are knotted in fifth place at 6-under-par 210.
Casey Martin fired a 4-under 68 on Saturday to get into a share of 10th place. Ben Ferguson (66), Marcus Jones (66), Jon Turcott (65), Jason Higton (70) and Brian Unk (71) joined Martin at minus-5.