Summary
To perform an Air Dash, once again you have two options:
- Double Tap the controller, while in the air, in the direction you wish to perform your Air Dash.
- Hold the controller in the direction you wish to Air Dash in and press any two of the three main Attack Buttons.
For characters with both an Air Dash and a Double Jump, however, you cannot double tap the controller in any of the Up directions to Air Dash in those directions because tapping Up causes you to Double Jump instead.
Air Dashing Freedom
Air Dashes are very powerful tools because they give access to very quick and sudden attacks and , like Double Jumping, also allow character to alter their jump arcs, providing them with significantly improved mobility in the air to attack from different angles or to escape traps from the opponent.
You can perform an Air Dash during a regular Jump or Super Jump pretty much at any point of the Jump. So you can Air Dash right when you leave the ground, at the height of your Jump, or even right before you are about to land from a Jump. Also, for the most part, Air Dashes count as Special Moves so you can also cancel airborne Normal Moves into Air Dashes. However, there are some odd exceptions to that rule where a Normal Attack in the air can only be canceled by an Air Dash and not a Special Move. A perfect example of this move is Doctor Doom's Foot Dive: his Special attack in the air.
Note that when you perform an Air Dash, you preserve whatever state you started off in. So if you are in a Jumping State and perform an Air Dash, you stay in Jumping State, so you can still do things such as use a Cross-Over Assists. If you were in a Super Jumping State when you performed your Air Dash, you will not be able to perform a Cross-Over Assist.
Classes of Air Dashing
Air Dashing is only available to certain characters and, between the characters that do have Air Dashes, they can vary wildly. Typically, Air Dashing comes in two categories: 8-Way Air Dashes and 2-Way Air Dashes. So far, only five characters fall outside of those two categories: Chun Li, Spider-Man, Super-Skrull, Morrigan, and Amaterasu, but I will talk about them separately at the end of this section.
8-Way Air Dashing
The most powerful form of Air Dashing is the 8-Way Air Dash, which allows the character, while in the air, to dash in all 8 directions that you can hold the controller. The characters who possess an 8-Way Air Dash are:
- C. Viper
- Doctor Doom
- Dormammu
- Iron Man
- Magneto
- Phoenix
- Storm
- Trish
- Thor
2-Way Air Dashing
The 2-Way Air Dash allows you to Air Dash in only two directions: forward in a straight, horizontal line and backwards in a straight, horizontal line. The characters who possess 2-Way Air Dashes are:
- Dante
- Hsien-Ko
- Shuma Gorath
- Tron
- Viewtiful Joe
- Zero
Chun Li, Spider-Man, and Super-Skrull
For Chun Li, she can only Air Dash in one direction: forward and at a slight downward angle though the game just treats it as a Forward Air Dash, so you have to double-tap Toward on the controller or hold any of the three Toward directions when hitting two of the Attack Buttons. Spider-Man can also Air Dash forward, but he really does more of a web swing that goes in a slight arc in the air. You can double-tap Toward on the controller or hold any of the three Toward directions on the controller and press two of the Attack Buttons. Super-Skrull can only Air Dash in one direction as well, but unlike Chun Li his Air Dash goes forward at almost at a 45% angle, but is also treated as a Forward Air Dash.
Amaterasu
Amaterasu can Air Dash in 3 directions, but all of them are forward directions either perfectly horizontal, about 30 degrees upward, and about 30 degrees downward. Theses are treated as the three forward directions on the controller.
Morrigan
And finally, Morrigan has 4 Air Dashes: forward, up/forward, down/forward, and back. Her forward and back Air Dashes make her travel fairly horizontal and the controller can be held forward or back to make it travel farther. Air Dashing up/forward makes Morrigan travel in a quick, parabolic arc going up and then down, sort of in a wide upside-down "U" shape. Whether you hold the controller or not makes no difference: once you start the Air Dash, it will travel the full arc unless you interrupt it with a move in the air. When you Air Dash down/forward, Morrigan does the opposite: she goes in a parabolic arc going down then up, in the shape like the letter "U." At the end of that Air Dash, she gets "launched" almost as if coming off a swing and will slowly sail forward and back down. Again, there is no difference on whether you hold the joystick or not, she will complete the full arc unless you interrupt the Air Dash with a move. To perform the four different Air Dashes, just double tap the controller in one of the four directions—Toward, Up/Toward, Down/Toward, or Back—to perform that Air Dash. If you use the method of hitting two Attack Buttons to perform the Air Dash, holding Toward gets you the forward Air Dash, holding Up or Up/Toward gets you the up/forward Air Dash, holding Down or Down/Toward will get you the down/forward Air Dash, and holding Back, Up/Back, or Down/Back will get you the back Air Dash.
The Power of Air Dashing
Just like with ground Dashes, the main benefit of Air Dashing is that you can cancel Air Dashes with attacks mid Dash. This allows you to perform many surprise attacks by changing your Jump arc and attacking immediately in a different angle than your opponent was expecting.
Strategy Corner
The true power of Air Dashing is revealed through what has come to be known as "Triangle Jumping." The reason it's called this is because you are basically performing very quick jumps in the shape of a triangle by jumping forward and then immediately performing an Air Dash down and towards the opponent. This makes you land super quickly and if you 
out an attack on your way down, it means you can attack with a move that must be blocked 
in less than a second after leaving the ground, essentially giving you a super fast overhead attack.
Now, obviously you can Jump at any angle and Air Dash at any angle as well to make this effective. But it's one of the cornerstones to a good rushdown character ever since MvC2 where Storm and Magneto could Triangle Jump at you all day long. The trickiest thing is that you can air dash downwards in Marvel Vs. Capcom 3 and you'll land immediately, meaning you could skip the air attack all together and go into an immediate 
hitting move right when you land, creating the ultimate / mix-up.
In fact, many characters' Air Dashes are so fast in Marvel Vs. Capcom 3 that you can't even get off a stronger attack before you land! If Magneto, for example, performs an Air Dash as to the ground as possible, a Special Attack pressed during the Air Dash will come out too slowly and Magneto will land before the attack connects. Magneto, thus, has to either Air Dash 
from a higher height to give his Special Attack time to come out and hit the opponent or choose to Air Dash with a Light Attack, as that will be fast enough to come out before Magneto lands. Either method works fine, but the Light Attack version is much quicker and harder to block.
Triangle Jumps remain a very deadly weapon in Marvel Vs. Capcom 3 but are not easy to perform. However, if your character is capable of performing Triangle Jumps, you would be very much pressed to learn how to perform them to make your character as effective as possible.
However, unlike ground Dashing, where you can cancel Air Dashes with Crouches and Jumps, you cannot cancel an Air Dash in any other way. Therefore, when it is said that the weakness of an Air Dash is that you are committed to the Air Dash, there is no way around it this time. So if you perform an Air Dash and your opponent activates a Hyper Combo, you will be eating that Hyper Combo. There is no Jump Canceling to save you.
Air Dashes in Combos
As with Double Jumps, Air Dashing can also be used to extend Air Combos. Canceling a move into an Air Dash allows you to squeeze in more moves in the Air for extra damage. However, only two characters possess a wide array of Normal Moves in the air that can be canceled into an Air Dash for this to be useful: Magneto and Storm. Otherwise, the majority of the characters with Air Dashes cannot cancel any Normal Moves into an Air Dash.