The 23-year-old Pendrith, making his first PGA Tour start, is coming off his senior season at Kent State.
"I never expected that," said Pendrith, from Richmond Hill, Ontario. "I was playing good golf coming in here, so I just played golf. Five under is pretty nice. Everybody has some nerves. If you didn't have nerves on the first tee something would be wrong with you. But after I played the first hole, I felt calm and felt I should be here."
Stanley opened with a 5-under 30 on the back nine and played the front in even par.
"I love this golf course," said Stanley, the 2012 Phoenix Open winner. "It's similar to the one I grew up with in Washington, so I felt pretty comfortable. I hit the ball well on my first nine today. I was able to make a few of the long range putts that got the round going for me."
Watney had six birdies and two bogeys on the tree-lined layout.
"You're not playing behind the 8-ball to make the cut," Watney said. "It's only one round, but I'd much rather play well in the first round than not."
Two-time champion Jim Furyk and Canadian David Hearn shot 67, and Brandt Snedeker, the winner last year at Glen Abbey in Oakville, Ontario, had a 69.
"Any day you come off the course and shoot 3-under and feel you left a few out there, you feel it's a good day," Hearn said. "I'm not overly disappointed with the way I played. I hit the ball great from tee to green. If I can continue to do that for the rest of the week I know I'm going to play well."
Canadian Mike Weir, an eight-time PGA Tour winner making his 24th start in the event, had a 70.
DIVOTS: Dustin Johnson struggled to a 74, and John Daly had a 76. ... Pat Fletcher, born in England, was the last Canadian winner, taking the 1954 event at Point Grey in Vancouver, British Columbia. Carl Keffer is the only Canadian-born champion, winning in 1909 and 1914. Albert Murray, a Canadian also born in England, won in 1908 and 1913.