Capcom have confirmed that the upcoming 3DS port of Monster Hunter 3 won't have online co-operative play.
One of the bigger draws of the Wii original was the ability to meet with other players online and embark on co-operative quests together. Unfortunately the feature doesn't seem to be feasible on the 3DS portable edition, however there will be local wireless co-operative and content updates from Capcom online.
Monster Hunter Tri G will be be boosted in other areas, with an expanded solo campaign amongst various 3DS tidbits.
Via 3DSConnect.