This is Capcom we're talking about - the king of porting. Who's to say that they won't port MHP3rd to 3DS sometime next year? All they would have to do is add a few monsters and maybe a new weapon and location and the game would have similar sales to its PSP brethren.
And I don't see how MH4 would be PS3 exclusive.
Tag - "No trolling on my watch!"