Monster Hunter XX launches in Japan on March 18th, 2017 for 3DS but I'm calling it right now, this game is going to be on Switch and released the same day. It would be stupid to launch such a big 3DS game like 1 week before the Switch because your sales would be cut off potentially in week 2.
I think it's more likely Monster Hunter XX Switch is going to happen. Switch version will have HD graphics and 60 frames per second, which will launch on March 18th in Japan (day and date with the 3DS version). Just like the Wii U launched with that HD Monster Hunter port (HD graphics, 60 fps).
Capcom can't say that now because they are waiting for the Jan. 12th event at Nintendo's request. Having one for the Switch launch is kind of a no-brainer, plus is starts building that Monster Hunter audience on the system right at day 1.
I think there will be a few 3DS/Switch dual releases for a little while until Switch builds up its userbase and takes over.