Board Thread:Questions and Answers/@comment-151.224.253.3-20130301211413/@comment-84.133.8.117-20130307203131

It seems that MH3U on the 3DS will not allow you to play online. However, if you own a Wii U you will be able to download a program from the Wii U eshop that will allow online play on the 3DS as long as you are connected to the console.

Source:

http://www.nintendolife.com/news/2013/03/monster_hunter_3_ultimates_3ds_online_multiplayer_workaround_is_coming_to_europe