Marvel vs Capcom 3/Systems/Air Combos: Air Chain Combos Magic Series
To perform an Air Chain Combo, tap the buttons in the correct sequence of the Air Magic Series to chain the Air Normal Attacks one after another.
The Air Magic Series
All of the rules mentioned in the Ground Magic Series section apply to the Air Magic Series. You can skip buttons in the middle of the Magic Series, you can delay the timing of the canceling as long as the move connects, if the move whiffs you cannot chain it into another move, etc. etc.
However, the main difference is that pretty much every character has the same Magic Series in the air. So, appropriately enough, it is referred to as the Air Magic Series:
Most Unique Attacks that are performed during a Jump end the Magic Series immediately, such as Wolverine's dive kick from the air performed by pressing Down and Hard Attack. Again, there are always exceptions to this rule. For example, Iron Man has three different attacks with the Hard Attack Button in the air by holding Up, Down, or Left / Right / Neutral. While most characters can't actually chain multiple Hard Attacks in the air, Iron Man can with such sequences as chaining Down and Hard Attack into Up and Hard Attack. So, once again, experiment with your characters to find out exactly what is possible.
The Special Button
Also, don't forget that the Special Button in the air counts as a Normal Attack, so unlike Launchers on the ground performed by hitting the Special Button, attacks from the Special Button in the air have all the same properties as a Normal Attack so you can cancel it into Special Moves and such, except in the case of being used as an Air Combo Finisher after a Launcher (See Air Combo Finishers).
The Limit of Three
Like the ground-based Magic Series, there are some characters that possess a 3 move limit... but only while performing a regular Jump. So even though, while Super Jumping, a character can perform a Chain Combo of 6 moves in the air, a regular Jump only allows them 3 moves total.
For example, with Spencer, if you jump at a standing Sentinel, he can perform a Chain Combo of Light Attack to Light Attack and then Medium Attack. However, if he tries any more hits, nothing will happen. However, he can also do Light Attack into Medium Attack into Hard Attack. Or he can do Medium Attack to Medium Attack to Special. As long as he obeys the Magic Series, he can do any combination of 3 attacks while in regular Jump.
Again, this is very character dependent. Many characters can easily chain as many moves as they feel like in a regular Jump, so you'll have to experiment with your character to find out your limits.