A9s SB vs BU 3bet Pot

Preflop is fine. Flop-size is fine. I do not see much value in a Turnbet. If he is floating an A, you are most likely behind (AJ/AT). After his cb on the turn, he is on any kind of single pair hand. Since the flop is so dry and the only draw came in on the turn, a bluff by BU is unlikely. I like your smallish bet vs Qx and Kx hands + vs Ax it works as a blockbet. NH!

Preflop is good - you can 3B or you can fold it.
But I dont see any reason for bet on the flop - he could call AKo, AQos, KQo, QJs, JTs - hands what he could call flop bet.
If you are hitting good on the flop than better will be check-rise.

I wouldn't mind calling a BTN raise ether. You basically change A9s in a bluff with 3betting, there is a chance you play a big pot out of position and the hand can play well post-flop if you just call.

If you Cbet the flop you need to do it with strong hands as well. Otherwise you are predictable, but against a unknown it makes it less likely villain has reads on you.

As played I go for thin value on the river as well.

Cheers,
SDK1987