With 27 days left, let's review the basics of Marvel vs Capcom 3, starting with the default button layout:
A B C
E A1 A2
The controls for the game consist of three attack buttons: light attack, medium attack, and hard attack. The buttons for these attacks are known as A, B, and C. Note that there are not separate punch and kick buttons; each particular attack may be either a punch or a kick. The fourth attack button is Exchange, or E for short. Exchange performs a launcher if you're on the ground or a combo ender if you're in the air. Attacks chain together from A to B to C, then to E. An example of a simple combo would be A,B,C,E, superjump A,B,C,E. Some specials are performed with the E button. For example, most characters' flight mode is activated with QCB+E. Just as in MvC2, there are two assist buttons. Calling assists works exactly as it did in MvC2, but tagging out is now done by holding either assist button. Pressing both assist buttons performs a Team Hyper Combo, wherein your assist characters perform their supers at the same time as the point character.
Supers, or Hyper Combos if you prefer, are generally performed with a single motion and any two attack buttons. In other words, supers in Marvel require QCF+AB as opposed to Street Fighter IV's QCFx2+PPP. Special moves can be canceled into supers, just like in SF4. This allows you to do things like fire Deadpool's guns from full screen, then cancel into his gun super on hit for big damage. Delayed Hyper Combos (DHC's) are exactly as they were in MvC2. You can activate the point character's super, then input the next character's super during its animation to have that character tag in and immediately perform that super. DHC's are not only a great way to extend combos for more damage, but also a way to safely tag out your point character.
The main new addition to MvC3's fighting system is something called X-Factor. X-Factor can be performed only once per match by pressing A,B,C, and E simultaneously. It puts all of your remaining characters into X-Factor mode for a limited time. When in this mode, the characters glow red and deal more damage. Your point character heals and moves faster. Activating X-Factor also cancels whatever the character was doing, sort of like a FADC in SF4. You can do things like super, X-Factor, super again for massive damage. The length of time the X-Factor mode lasts depends on how many characters you have remaining. When you have all three, it lasts the least amount of time. When you're down to your last character, X-Factor lasts quite a while. Because of this, X-Factor adds a huge comeback potential to the game.
So, that takes care of the basics. If you have any specific control questions, post them in the comments below.