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!
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.
There are lots of different roles characters can actually fulfill as the Point Character in Marvel Vs. Capcom 3, so you'll have to craft your team accordingly to how you want your team to be used and what benefits the characters you have chosen the most. There are a lot of possible roles and many characters can actually act in more than one role at a time and can sometimes shift roles depending on the situation. Below are some of the most common examples of the types of Roles that members of your team can play.
The whole role of this Point Character is to be an offensive force. They are not meant to be played defensive nor are they meant to runaway from the opponent and defeat them through attrition. They must have great offensive options and mobility. Wolverine and Chun Li are good examples of characters that can fill this role well.
Some characters are better off being defensive and trying to entice the opponent to break your defense as much as possible. And because the opponent gets frustrated trying to break into the defense, they open themselves up to combos and damage much easier. M.O.D.O.K. and Arthur are good characters to be used for the Keepaway role.
Battery characters are characters who are good at fighting without Hyper Meter. They can adapt to many situations and are ideal to be used alongside characters that need Hyper Meter to be effective. Magneto is a perfect example of this type of character as he can play offense or defense and be very effective without ever using Hyper Meter. This allows him to build up a good amount of Meter for a teammate to come in and wreck shop. Wesker is another good example of this as he can get in easily with Teleports and run away well with his Samurai Edge gunshot. And his Hyper Meter is not what makes him effective.
These are the characters who are really effective as Point Characters and can rushdown well, but need a lot of Hyper Meter to do so. These characters are usually best paired with some Battery characters so the Battery Character can build up the Hyper Meter and then you can switch to the Damage Dealer that needs the Hyper Meter. Felicia is a great example of this sort of character as she is by far more of an effective character so long as she continually has meter to activate her Kitty's Helper Hyper Combo. C. Viper is another example of this as she needs a lot of Hyper Meter to be really effective, as her EX Burn Kicks and EX Thunder Knuckles are very potent in her offense, but cost one Level of Hyper Meter every time she uses them.
These type of characters are good for countering opponents in the rushdown role. They don't land hits easily but, when they do, you can drain half of the opponent's life with little or no effort. Rushdown characters need to land multiple combos on them to achieve the same damage they do in one combo. That means all they need to rely on is one mistake from the opponent, and they can deal a huge chunk of damage. Hulk and Haggar are examples of these kinds of characters but, oftentimes, these characters lose pretty badly to keepaway characters.
So this is a starting point to understanding how to properly balance out your team. If you create a team of nothing but rushdown characters, a team of Haggar, Sentinel, and Hulk will destroy you pretty badly. But then a team of M.O.D.O.K., Chris, and Dormammu could destroy that team! So really you may want to consider having characters on your team that can fulfill multiplt roles to cover each other's weaknesses.
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.