Rob McMillan matched Pettitt's 68 to move into second place at 5-under-par 208. Steve Scott is one stroke further back at minus-4.
Pettitt got off to a quick start to his third round with a birdie at the par-4 first of Whitney Oaks Golf Course. After four straight pars, he dropped in another birdie at the sixth.
Around the turn, Pettitt moved to minus-8 with a birdie at the 10th. He stumbled to a bogey at the 12th, but erased that error with a birdie on No. 14. He parred in to maintain his lead.
Pettitt will look to pick up his first win on the Canadian Tour Sunday. His best finish on tour was a tie for ninth at the 2002 MTS Classic.
McMillan wrapped birdies at the first and third around a bogey on No. 2. He dropped another stroke on the sixth. He ran off three straight birdies from the 14th to claim second place heading to the final round.
Andrew Barnes, Matt Hansen, David Hearn, Wes Heffernan, Kyle Kovacs and Brad Sutterfield share fourth place at 1-under-par 212.