MHP3rdHD doesn't make sense because MHP3rd already runs on Vita, and more resolution vs filtered emulation wouldn't be enough.
It would be too late for a direct MH3G port. But in this case it would be better later than never.
A spin-off like these released in PSP with the MH cats would be pointless.
So I bet it would be MH3G HD (Vita, PS3) :
Think MH3G + some new content (some few extra missions, some few new monsters), online crossplatform play like in MH3PHD. 3D for the PS3 version and a Near based gift feature, to send weapons, armors, items or even custom missions in the Vita version. I doubt that if Capcom and Sony did all this work for this crossplatform feature / porting for only 1 game. They like to port everything, and since MH3G uses MT Framework Mobile (already shown in Vita, and running all HD version features), it wouldn't be so hard or expensive to up port it to Vita and PS3.
Considering the typical timed exclusives, I think it will be released like 6 months after the 3DS release, not a year after.
I think MH4 will be multiplatform and will be released in at least Vita and 3DS (who knows, maybe PS3 and Wii too). But I think H2 2012 is too soon for it. At that time they still can be selling MH3G at a reduced price, and a cheap port like I mentioned.
Yeah man, I think you're right! I remember they made up their mind with that RE4 gamecube exclusivity deal too!
RE4 was announced as one of the 'Big 5 Capcom exclusives for Gamecube', like the PS3 MH3.
MHTri, MH3G and MH4 never have been mentioned as exclusive.