I think 3 betting is worst option vs 4x open here with ATo, if he's a fish then 4x is a sure bet size tell, if he's not a fish then I don't like folding his worse Ax out with mediocre Ax. I'd also X turn or flop. Too thin.
I am going to assume that you are right in saying this player is a fish
Like a flat more but i am fine taking taking the betting lead against a fish with a decent hand. After that I think betting flop and turn is pretty standard. You could defiantly be beat but i expect a fish to call with any A and medium pocket pairs. I think i like a smallish river bet around the 21-24 dollar range to get value from Ax Kx and Qx as well to minimize losses a bit if you are beat.