The fans of Monster Hunter 3 Ultimate in the West have gotten quite a bit of good news recently. There is a large selection of pre-order bonuses from various retailers they may choose from as well as the recent announcement that NA and EU servers would be merged via a software update scheduled for April.
A few weeks ago the Nintendo Direct for Japan highlighted the ability that Capcom is providing for a player using a 3DS to be able to play online. Initially this was only mentioned for Japan, however a moderator on Capcom's Unity Forum confirmed that this function is coming to EU and that NA should follow.
Previously 3DS owners would only be able to engage in multiplayer quests on a local level. A Wii U console could join together up to three 3DS units, providing they were in the same room. Now, through the use of a Wii U, a free downloadable application, and a LAN adapter for the Wii U, owners of the 3DS have access to the online city that is part of MH3U. There are some limitations with this setup.
While you do not need the Wii U version of MH3U for this system to work, if you do have it you will not be able to play the Wii U version while the console is relaying gameplay from the online servers to the 3DS unit.
The 3DS will not have the ability to create cities online, only join one that has already been created by a player on a Wii U system. Once in the city, they should be able to post quests as normal.
Only one 3DS unit can be connect to the internet through this setup.
One final piece of information to keep in mind is shown in the picture above and may be understood by most, but perhaps not by all. The reason the LAN adapter is needed is to provide the Wii U console with a hardwired connection to the internet. The WiFi chip in the console is used to communicate with the 3DS unit, so it can no longer connect to the wireless router, which most of us use to connect the console to the internet. You will need a physical ethernet connection at your console to plug into the LAN adapter.
As mentioned, this has been confirmed for EU territories and we see no reason why the setup will not be available for players in NA as well.