362/365 is actually what IM2's numbers would roughly translate into with ticket price inflation and 3D boost (if you go by the 15% thumb rule).
Now, even if I think that the 400M mark is far from being impossible to reach, there are several things that should be taken into consideration.
First and foremost IM3 will come after a movie that already sold 5 million less tickets in the domestic market than its predecessor wich is widely considered as the weak link of the MCU Phase 1 movies. In addition, the alleged "Avengers boost" on solo franchises is not at all certain at this time and even if it plays a role, in the end, it'll be no more than a marginal boost to the OWE numbers ,after wich the movie will have to perform on its own merits.
Just to get things straight, I don't say that 400M domestically won't happen, just that I think it's a mistake to take for granted that a sequel will necessarily make more than the previous installement in the franchise. To reach the 400M mark, IM3 will have to get back to a 2008 Iron Man like attendance wich is, on a decaying market (if you reason in terms of tickets sold), a challenge of its own kind.