An AC enters in combat with 2 ftr on board.
Another ftr comes from land and arrives here with its 4th movement, pretending to land on the carrier assuming that at least one fighter will be lost in the battle.
Is it a legal move?
No, it’s not legal. Â You must be able to plan a landing-spot for every fighter during your Combat Movement phase, assuming that all fighters will survive their combats.
However, in the scenario you provide, you may have a way to get that 3rd fighter to the fray.
The fighters on board the AC actually did not arrive at the combat by “riding” the AC to it. Â They took off from the AC at the start of Combat Movement and flew to the combat SZ using their own movement points.
Therefore, those fighters still have movement points left with which to fly to a safe landing spot after the combat. (At least 2, perhaps more). Â If there is such a safe landing spot available for at least one of them, then you can legally bring that 3rd fighter to the fight, and land it on the AC.