I have been playing a lot MH3U on Wii U, and also my son... He has a 3DS, and I'd like to play online quest with him, and another players around. If i buy a 3DS cartdrige of this game, we can join online quests simultaneosly? For example, I play on Wii U, and he plays on 3DS, at the same time, in the same online quest, with other two any players?
You can't. The 3DS version of Monster Hunter 3 Ultimate can only access online capabilities when a certain app is run from the Wii U. The only way to play multiplayer locally would be through the use of Monster Hunter 4 Ultimate with multiple 3DS systems.