Despatie, of Laval, Que., qualified ninth with 458.55 points. Imbeau-Dulac (449.30), from St-Lazare, Que., moved following a 12th-place finish .
"I was a bit annoyed at the beginning, but it got better as time went on. In the semifinal I am keeping the same dives. I did not make any big mistakes," Despatie said. "I am never the person who arrives in the preliminaries and dives the most impressive dive."
Ilya Zakharov of Russia led the men in the preliminaries, with He Chong of China close behind in second during a competition marked by pratfalls.
Zakharov totaled 507.65 points during the six rounds. He earlier won a silver medal in synchronized three-metre with partner Evgeny Kuznetsov, and was the silver medalist in the individual event at last year's world championships.
He, who was ninth after his opening dive, totaled 500.90 while opening defense of his Olympic title.
Troy Dumais of the U.S. was third at 486.60 after rallying from sixth in the fourth round.
Dumais has never finished higher than sixth in three-metre at three previous Olympics. He finally earned his first Olympic medal in springboard synchro with partner Kristian Ipsen.
Stephan Feck of Germany dropped out of the competition after the third round. He lost his grip on his right leg doing a forward 3 1/2 somersault pike and landed on his back, smacking the water as the crowd gasped. He received all zeros from the judges. Feck did his third dive and then withdrew, with officials saying he felt faint.
Jack Laugher of Britain hushed the home crowd when he missed his last dive. He appeared to slip off the edge of the springboard after launching into his jump and earned all zeros. Laugher finished 27th.