A7-straight against flush

If he is loose aggressive you chose the worst hand to 3bet him with. Your hand is easily dominated in 3bet pots. If you flop an ace you only get action when you are crushed. If he folds to 3bet 80%+ of the time then 3bet is fine as a pure bluff with a blocker. But against someone who opens a lot and doesn't fold to 3bet this is the worst hand to 3bet with. Fold preflop.

Against loose opponent betting once on the flop and not barreling all three streets is also a mistake. LAGs will float you more often than not, they will check/raise more often than not. Betting once against them is playing into their field. I would bet the turn here (but not all-in). Make the sizing for almost all of your chips so that he has to click the raise all-in button because it will it tougher for him to get it in with 2nd pair type hands. It is much easier to click call button than the raise button psychologically.

Pre-flop is even against a LAG a fold. No need to take a variance line with 3betting and you are also committing yourself against the BB if he shoves.

Cheers,
SDK1987