The three companies are all authorized Canon dealers. Generally, Canon will service any new product bought from a authorized seller, just be sure you have the receipt. They have been said to service gray market from authorized dealers as well, but right now, prices are lower on USA models.
The warranty card does not give you a warranty, its misnamed, since all it does is register you as a buyer. Canon asks for the receipt for warranty service but so far, they will service any camera within a year of sale. They have been good about that.