Timedog said:
-when it is best to use certain combos (why choose one combo over another in a certain situation)
you use whatever works, with the endgoal usually either being damage/kill, meter buiding, or positioning for oki/resets. of course, you're not beating up on a stationary dummy, so you need combos that work on standing opponents, crouching opponents, ground-to-air via anti-air, air-to-air i.e. jumploops, and very important especially for this game getting a combo off a (air)throw. also, depending on your team, its also a good idea to have combos that hit confirm off your assists and combos that work after a tac. also remember, your combos might be different in xfactor lvl3. once you get all those down, you might start looking into corner-specific combos that do big damage.
-what type of assist you would need to have to accomplish certain combos
easiest combo extending assists are ones that either otg, or ones that create long enough hitstun to combo from your own otg.
-what assists to choose with your characters
depends what you want or what your point character is trying to achieve. there are a variety of assists used for different purposes, with usually the better assists that can be used for more than 1 purpose. generally though, i like to always have an assist that can be used to make blockstrings safe(r). which can either allow you to dash/tri-dash back in for more pressure, or to back off and throw a projectile or just reset the situation.
-ways of deciding team order
-different methodologies for deciding your team (Actually the Viscant tutorial for MvC3 Vanilla goes over the Battery/Assist/Anchor method, but I think there are other methods to)
-character specific ways of getting in/mixing up
-how/when to use xfactor and do it intelligently
-stuff that you should never/rarely use with a character
-effective ways of practicing
-ways to defend against specific characters (what to look out for when your opponent chooses a certain character)
-etc
too lazy atm to answer the rest, but you shouldn't be thinking purely of combos. you need to know your basic buttons and what beats what, and you also need to be able to move around the screen effectively, i.e. wavedashing, tridashing. if your character has a very specific touch-of-death combo that only works in the corner and only on standing opponents, what's the point of practicing that sole combo if you can't even get to meet the conditions for that combo to occur in a real match?