Trailturtle, the CO Community Manager, has said outright that all three games are running on the same physical box. I haven't seen anyone reveal specs on said box, but that puts a bit of a strain on the hardware, given what I can only imagine is tens of thousands of users, between the three games.
Maybe I'm overly optimistic with regards to the popularity of the other two games, but I routinely end up in the 4th or 5th instance of whatever zone of Millennium City I'm in, and the "common areas" are always downright bustling in the other two games, though I don't go into them often enough to think to look at which instance of the zone I'm in.