Stephen Gangluff posted a 1-under 69 on Saturday and is alone in third place at minus-3. Matt Hansen and Anders Hultman each carded rounds of even-par 70 and are the only other players under par at 1-under-par 209.
Tomasulo mixed two bogeys and a birdie over his front nine and lost the lead to Harris. He birdied the 10th, but dropped a shot at the 12th to fall to 4-under par for the championship.
The Canadian Tour rookie caught fire down the stretch. He birdied three of his last five holes, including No. 18, to draw even with Harris at 7-under par.
'I made some bad swings early, but I kept my composure and was able to stay in contention,' said Tomasulo. 'In the past, I've been down on myself after a bad shot. Today I kept it together. I always look forward to Sundays, especially when I am in contention. It should be a lot of fun.'
Harris played differently than his fellow co-leader. Harris tallied four birdies on his front nine, then birdied two of his first four on the back nine.
He held the lead at this point, but bogeyed the par-5 14th, a hole he birdied in each of the first two rounds. Harris parred his remaining holes as he goes for his first win on the Canadian Tour.
'I really got off to a quick start, settled in and just kept it going,' said Harris. 'I was hitting solid shots all day long and, well, here I am now. Hopefully I can close the deal Sunday.'
Kevin Senecal fired a 3-under 67 and is alone in sixth place at even-par 210.
Rob Oppenheim (72), Jaime Gomez (69) and Jon Turcott (69) share seventh place at 1-over-par 211.