Barbara Jardin of Notre-Dame-de-Grace, Que., Samantha Cheverton of Lachine, Que., Toronto's Amanda Reason and Brittany MacLean of Etobicoke, Ont., swam to an impressive time of 7:50.84, just behind Australia (7:49.44) and the United States (7:50.75).
The relay final is set for later Wednesday, at 4:04 p.m. ET.
Meanwhile, it was a good morning in the pool for Canada's individual swimmers.
Julia Wilkinson of Stratford, Ont. advanced to the semifinals of the women's 100-metre freestyle, swimming to a third-place finish in her heat with a time of 54.16, good for ninth overall. American Missy Franklin, who has already picked up gold and bronze medals in London, also qualified for the semis with a time of 54.26.
Wilkinson will be in the pool for the semis around 2:40 p.m. ET.
Toronto's Tobias Oriwol booked his spot in the semis of the men's 200-metre backstroke with a 1:58.06 finish in Wednesday's heats. He'll be joined in the semis by American swimming stars Tyler Clary, who finished with the best time in the heats with a time of 1:56.24, and Ryan Lochte, who was close behind in second with 1:56.36.
Those semifinals will start just before 3 p.m. ET.
Martha McCabe's 2:26.39 was good enough for the Torontonian to book a spot in the next round in the women's 200-metre breaststroke, but teammate Tara Van Beilen of Oakville, Ont., won't be joining her. Her time of 2:27.70 wasn't fast enough to make it into the semifinals.
American star looking for another gold
Michael Phelps opened the defence of his title in the 200-metre individual medley, a day after becoming the most decorated Olympian ever with his 19th career medal.
Phelps' time of 1:58.24 seconds was good for fourth-fastest in the preliminaries. Teammate Ryan Lochte was second-quickest at 1:58.03, a short time after he advanced in the 200 backstroke heats.
Canadian Andrew Ford qualified for the semi with a time of 2:00.28.
"It was kind of hard stepping up in the morning," he said. "All that celebrating takes a lot of energy out of you. All I had to do was get a lane for tonight, and that's what I did."
The 200 IM final is Thursday.