What you saw Axe do is a double bair: first bair on the shorthop and the second bair on the double jump. For my fingers, the most reliable method to accomplish this is by using the control stick to jump and the c-stick to bair, but for more variation you should learn how to do it using face buttons, especially if you want varying degrees of aerial drift.
It's really important to to get the first bair out as close to the first actionable frame as possible, and then double jump immediately after the first bair animation completes. You don't have to be frame perfect, but there aren't many frames of leniency.
The last cool thing is that after the second bair, you have enough time to nair with an active hitbox.
2 bairs + 1 nair = triple aerial.