If you have the 3DS and Wii U versions of Monster Hunter 3 Ultimate, Capcom has announced that the transfer program allowing you to move your progress and data between both systems is now out in the 3DS eShop.
A sister application, the Monster Hunter 3 Ultimate Packet Relay Tool has also been released in the Wii U eShop. It allows 3DS users to play with others online by connecting through a Wii U, provided that Wii U uses a separate wired LAN adapter and another Wii U player is hosting the game.
While the Wii U is running the Packet Relay Tool, the system can’t be used to play anything else. On the bright side, you don’t need a Wii U copy of Monster Hunter 3 Ultimate to use it.
Capcom’s official statement:
Fans who own both consoles, as well as a copy of the game for each system, can use the Monster Hunter™ 3 Ultimate Data Transfer Program to transfer their progress from one console to the other and vice versa for a seamless experience between hunting on the go and in front of the TV. This application will be available on Nintendo eShop on Nintendo 3DS.
The Monster Hunter 3 Ultimate Packet Relay Tool for Nintendo 3DS is an application that can be downloaded for free from the Nintendo eShop on Wii U in order to play the title online on your Nintendo 3DS, via a Wii U console connected to the internet.
This application allows players of Monster Hunter 3 Ultimate on Nintendo 3DS to play the game online by using a Wii U console connected to the Internet via the Wii LAN Adapter or Wii U LAN Adapter (to be sold separately). Using this set-up, one Nintendo 3DS system can connect to the Wii U console, and play Monster Hunter 3 Ultimate online with other players, provided the online game is hosted by another player playing the game on Wii U. Please note that while the Wii U console is providing this function for the Nintendo 3DS system, it is not possible to simultaneously use the Wii U console for playing Wii U software or to connect locally to another Nintendo 3DS system. A copy of Monster Hunter 3 Ultimate for Wii U is not required.