Capcom Clarifies Monster Hunter 3G Internet Play Situation
No online play for 3DS title.
If you were still holding out for a surprise announcement regarding online play for Monster Hunter 3G, it's time to give up. Capcom has posted a response at its support page confirming that the game will not allow for cooperative online play.
As detailed at the support page, the game uses the internet for download event quests. If you want to play cooperatively with other hunters, you'll need to do so locally.
This ad-hoc only play scheme is similar to the PSP Monster Hunter games, but different from the online compatible console games.