Mexico and the US easily have the facilities to host on their own. Canada is at least as qualified to host it solo as South Africa or Qatar. I would understand a three-country bid if it were Mexico + two countries from Central America or the Caribbean. I could even understand a Canada + US bid. Having it span three huge countries on a huge continent seems silly.
I would like to see Australia-New Zealand, Portugal-Spain, and Peru-Chile-Ecuador bids though, plus a Scandinavian bid.