I don't expect Marvel vs. Capcom 3 to come out for a long time, if ever.
One of the reasons why so many Vs games were pumped out so quickly those years was that almost all the sprites could be reused. For MVC3, they are going to have to start from scratch, which probably is not a good thing. Since the chances of a MVC4 might not be there, these sprites are going to be used for one game only, which might not justify the expenses to create the new sprites. If they use the same 1994-1996 sprites, nobody is going to bother with it, unless it is a PSP or DS exclusive.