Monster Hunter 3G will not have online game play. 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
Monster Hunter 3G will be one of the first 3DS titles to support the new Slide Pad dual analog accessory. It’s set for release this December in Japan.