While a ship is under construction, you can set the air wings that it launches with by clicking the button in the lower right corner of the ship picture (a blue circle with a plane in it).
You can easily find all the carrier air wings in the air groups list (the army / navy / air buttons in the upper right of the UI) and edit their sizes as you see fit. Aircraft get issued (or withdrawn) to match the size you set. But you do have to do that for each wing.
I don't know of a way to set a global policy like "all carriers shall have 1/3 FTR, 1/3 NAV, and 1/3 CAS", and update all carriers existing in the navy with a single click.