Details coming out of the Wii U press event in New York indicate that Capcom is bringing Monster Hunter 3 Ultimate to North America on both the 3DS and the Wii U in March of 2013.
Reggie Fils-Aime confirmed that Capcom plans to offer online multiplayer and local co-op play, as well as continuation of games between 3DS and Wii U.
That's all we know for now, folks, more info to follow, I'm sure.
Correction: It is called "Ultimate", not 3G. Article has been updated.