The diagram tries to explain

While today's 3DS Nintendo Direct from Japan showed a variety of interesting upcoming games, there was a surprise announcement of a new peripheral that will allow those with Monster Hunter 3 G on 3DS in Japan to take their local multiplayer games online for the first time. All you need is a Wii U and a special LAN adapter.

The wired accessory plugs into the Wii U, and a relay download tool is also required. You can then connect for online play on the 3DS version of the game, though there are a couple of limitations at this stage.

  • The Wii U version of the game can't be played at the same time that the relay tool is being used for the 3DS version.
  • You cannot create a four player lobby, but only join a lobby setup by someone with the Wii U version.

At the moment this is only confirmed for Japan, though with Monster Hunter 3 Ultimate on 3DS also lacking online multiplayer, it'll be interesting to see if the same solution will be available in the West. It may not be perfect, but it will allow owners of just the 3DS game, when at home, to enjoy some online play. Though you will need that adapter and a Wii U.

So, what do you think of this? Is it something you'd consider using if it made it outside of Japan?