User blog comment:Plexioth- xD/Monster Hunter 3 Ultimate/@comment-4421067-20130221201415/@comment-4421067-20130222071634

here is the source http://www.nintendolife.com/news/2013/02/monster_hunter_3_g_on_3ds_to_go_online_via_wii_u_adapter

so in order to play online on the 3DS you need to:

1. get the adapter

2. run the application on the WiiU

i don't see it quite viable unless you really prefer the 3DS version over the WiiU one.