Marvel vs Capcom 3/Systems/Attacking: Chain Combos Magic Series
To perform a Chain Combo, tap the buttons in the correct sequence of your character's Magic Series to chain Normal Attacks one after another.
It's good to familiarize yourself with your character's Normal Attacks so you know which moves to use and where. But it's better to really get to know your character's Chain Combos because this is where you'll be starting the majority of your offense.
What makes Chain Combos so valuable is that you can start with your quicker attacks, which are safer to throw out because they are harder to punish due to their speed, and, if they connect on the opponent, you can work your way up to the higher damaging moves through a Chain Combo. So while She-Hulk's best poke, her Crouching Light Attack, only does 48,000 damage on its own, you can easily chain that move into a Medium Attack and then into a Hard Attack right into a Launcher and then go for an Air Combo afterwards for huge damage.
Note that Normal Attacks cannot be chained unless they make contact with the opponent, whether it hits or is blocked. On whiff, you cannot cancel a Normal Attack into another Normal Attack. Also, if the move you start with connects on the opponent, you can chain into the next move at any further point in that move's animation. For example, if you perform Ryu's Standing Hard Attack, you can chain that into a Crouching Hard Attack immediately when the Standing move hits or you can wait a while and let the Hard Attack continue animating a bit before you chain into the Crouching Hard Attack. Any point after the move makes contact is chainable, though as with everything there are always a few exceptions.
The Magic Series
Every character has a rule set they must abide to that dictates what Normal Moves can be chained into which other Normal Moves: the Magic Series. And each character usually has a Magic Series mostly unique to themselves, especially given the fact that many of the Unique Attacks have their own idiosyncrasies on how they can be used in Chain Combos. For example, a character like Wolverine has intense flexibility: he can chain a Standing Light Attack into a Crouching Light Attack into a Standing Medium Attack into a Crouching Medium Attack and finally into a Standing Hard Attack. However, other characters like Hulk and Dormammu can only perform a Chain Combo of two moves: a Light Attack into either a Medium or Hard Attack. Then some characters like Sentinel have a similarly limited Magic Series but have an exception where he can also chain Standing Heavy into Crouching Heavy. It's really different for every character so take the time to really figure out what the sequence is.
The three main types of Magic Series are the Zig-Zag Magic Series, the Strong Magic Series, and the Light Start Magic Series.
Zig-Zag Magic Series
In the Zig-Zag Magic Series, the order which you can perform your moves is as follows:
Stronger Magic Series
In the Stronger Magic Series, the order which you can perform you moves is as follows:
Light Start Magic Series
And in the last Magic Series, the Light Start Magic Series, you can perform your moves in the following order:
If a Magic Series is more than two buttons long, you can skip any of the middle buttons and jump straight to later buttons in the series. However, you can never go backwards in a series. For example, Wolverine has the Zig-Zag Magic Series listed above. But he does have to go in that specific order. He can choose to just do a Standing Light Attack -> Standing Medium Attack -> Crouching Medium Attack -> Crouching Hard Attack, skipping the Crouching Light Attack and Standing Hard Attack all together. However, he cannot do a Standing Light Attack -> Standing Hard Attack and then back to a Crouching Medium Attack. You can only progress forward in the series.
Once again, though everyone's Magic Series follow those three patterns closely, everyone has exceptions to the rule that will deviate from the expected pattern.
The Limit of Three
Although the Magic Series for a character can be several moves long, there are some characters that possess a 3 move limit. So if you look at a character's Magic Series and notice that the longest sequence of moves in the Magic Series is, say, 7 moves long, you still are not able to actually get all 7 moves out in a ground Chain Combo: 3 is the most you can do.
For example, with Captain America, he has a Stronger Magic Series. But he also has a Unique Attack that can come out if he hit Standing Medium Attack twice: he performs a straight kick once to the chest and, upon pressing Medium Attack again, he performs another straight kick to the stomach. However, performing this sequence actually counts as two separate moves in the Magic Series and Captain America is one of the characters that possess the 3 move limit. So if you perform a Dashing Light Attack and chain into Standing Medium Attack and chain into another Standing Medium Attack, that actually ends his Chain Combo thanks to the 3 move limit! The second Medium Attack is fully chainable into Standing Hard Attack, you just won't be able to do it because of the limit. If you actually walk up to the opponent and just do Standing Medium Attack into Standing Medium Attack, you can then chain into Standing Hard Attack without any issues because you are still within the 3 move limit by skipping the Light Attack at the front.
Again, this is very character dependent. Many characters can easily chain as many moves as they feel like in a Chain Combo so long as they reach the opponent.
Also, remember, Launchers do not count as Normal Attacks, so they are not included in the limit of 3 attacks. In any of those examples with Captain America above, you can buffer the last hit of the Chain Combo into the Launcher.