I don't know why 3betting here, with 3 people already in the pot, even crossed your mind.
IMO, with a hand like A9s OOP you are hoping to hit 2-pr or a FD on the flop.
This is a terrible flop for your hand 4-way, so either throw out a probe bet and try to take it down right now, or check and hope everyone else checks then you can try to take it down on a blank turn card.

Basically, you do not want to invest very much $$$ at all in this pot; get to showdown as cheaply as possible is the goal.