The only legit possibility is Toronto. There is plenty of legit reasons why it would never happen, but if an ownership group did come along with ~2 billion dollars to spend on a expansion fee, building an arena (in a place that makes sense, NOT in a suburb like Markham), compensation to the Leafs for encroaching on their territory, and the NHL was actually interested in having an additional team in the GTA, I think it could work.
If it actually happened ~5 years ago I don't think they would have had a ton of issues winning over a decent chunk of the city as it seemed like a significant portion of the Leaf fan base was on the brink of revolt. With the Leafs' current revival though any new team would have to work harder to carve out a fan base.
Well it's not 3 times the population, but your not completely out to lunch.
As others have said, the Toronto metro has ~6 million and the Montreal metro has ~4 million, but unlike Montreal, Toronto is surrounded by other large (at least by Canadian standards) CMAs. Once you factor in places like Oshawa, Barrie, Kitchener, Guelph and Hamilton the region has another 2+ million people within ~1 hour of Toronto proper.