you can't really go by MVC2 sales, or any of the vs. games to date. first off, none of the psone versus games were good ports. the saturn had good ports but didn't have much of a userbase in the states. the dreamcast version was a perfect port but again, no userbase. the ps2/xbox version came out a couple years later but it was a barebones port and while accurate to the arcade version there was just no excitement. capcom didn't ship many units either.
make no mistake that mvc3 is going to sell well. there's really nothing else coming out, its 10 years in the making, and the game actually looks pretty good. capcom has done a really good job of building up the hype and people are ready for it. the special editions are sold out everywhere and pre-orders are high