The issue with having 3 attack buttons would be fine if you still had all your attacks and there was a definitive standard on how your perform certain attacks. For example, if as is, all three buttons were punches, and when I say punches I mean for example:
Ryu's Jab Strong Fierce with their respective animations, then all the D+attacks would be ducking kicks, and D+B or forward would be ducking punches, B+standing would kicks, I think it'd be ok. They could get away with having three buttons this way. But instead, you have like, a standing jab, standing forward, standing fierce, then F+ what is his MK would be the overhead, B or F+ Fierce is RH, D+F is trip, D+F+Fierce is launcher. Why is there a launcher button? In TvC there was another button as well, and a special type attack that required combination of buttons. That is a massively ridiculous waste, when you could have the MvC2 layout and retain all the normals. Hell why not have a pro layout that uses all 6 buttons plus those two extra buttons PS3 and 360 have. In TvC I also hated how you could launch off trips for free. Makes no sense other than to fill in missing comboing. This still exists in MvC3. You could combo off trips in previous games but they removed that since it was broken.
A big problem I had with MvC2 was you lose a number of naked normals with characters through the 4 button + assists system. The animations still exist in the game, but you only see them in combos. It ruins some characters, and I always like to point out how Venom gets nerfed because his ducking MP was so good in MvC. I think one of the only standard replacement methods is for launchers that were standalone attacks, like Jin for example, D+F+Fierce = his standing MP from MvC, same for Venom. For Cable it gets confusing at times because he has F+moves and D+F+moves that are different than standing and ducking. There are plenty of other characters that lose out. But in TvC, 30% of the normals are straight up gone. It's an unnecessary level of dumbing down.