Team Play
Marvel Vs. Capcom 3, just like its predecessor Marvel Vs. Capcom 2 (MvC2), is a game of three-on-three unlike a more typical fighting game such as Super Street Fighter IV. And unlike King of Fighters XII, where you pick three characters and use them one at a time, you will essentially be using all three characters at once, switching between them mid-fight and calling them out to aide each other in various ways.
And this is exactly what gives Marvel Vs. Capcom 3 its distinct flavor. It's not simply about picking your three favorite characters and just going with them, it becomes more about constructing a proper team whose abilities complement each other well. If you have a rushdown character like X-23, it may benefit you to have a teammate who can cover your attack from a full screen away. If you are a defensive character like M.O.D.O.K., you might be better off with a partner who is capable of building shields for you to prevent the opponent from getting in on you. It goes on and on and on. So even though there are only 36 characters in the game for now, in actuality there are a possible 7140 teams you can actually create! And that is not even taking into account Assist Types nor order, which ups the possibilities by a gigantic amount!
Character Roles
Once you chosen your team, the three characters will all fulfill a specific "role" at the start of the match. By default, the first character selected will be the initial "active character" in battle. This role is referred to as the "Point Character." The other two team members will be relegated to being "helper characters" that will aide the Point Character. The second character you've chosen will be the relegated to the role of "Assist Character 1" and be assigned to the "Assist 1" Button. And the third character selected will be the relegated to the role of "Assist Character 2" and be assigned to the "Assist 2" Button.
However, during the course of battle, the characters will be swapping and switching roles at a rapid rate. But just be aware that the active character that you are directly controlling will always be the Point Character and the other two characters will be Assist Character 1 and Assist Character 2 and assigned to the appropriate buttons.
Strategy Corner
TEAM ROLES FROM A STRATEGIC STANDPOINT
Swapping Starting Roles
Notice how "by default" was said a couple of paragraphs before this one. The reason for this is because, right at the beginning of the match when your team is being "introduced," you can actually swap the roles of two characters immediately, regardless of the order you picked them during the character selection screen. If you hold one of the Assist Buttons down before the intros finish, the Point Character and the Assist Character assigned to that button will switch roles immediately.
This can be useful for match-up reasons. If you see your opponent has selected as their Point Character someone who gives your Point Character a really rough match, you may prefer using one of your other teammates as the Point Character to start with instead. It's a nice bit of flexibility that allows players to pick their team the same way every time and adjust it on the fly. Think of it just like calling an audible in football.