With the feesh in between you and the nit kinda blows, but you are obviously going to 3bet here. Sizing is the only problem imo. He's probly not gonna be calling a shove if he has a draw, maybe with an overpair, but even thats doubtful. The fish may or may not come along with the 3bet, probly will if he has FD, probly not if just a one pair hand. I think a range for the BTN is probly 22+, ATo, KTo, QTo, ATs, KTs, QTs, and probly some pairs and FDs, FDs and straight draws, etc.

He will probly call a large raise with the majority of these hands, so the question is still size. Would raising to $12-15 be too much? Pot is $7.25 after BTN raises, if you raise to $12 it will be $19 and he has to call (if UTG folds) and he has to call $9. So that seems maybe too small? Sorry I'm rambling lol, just trying to spitball and see what would be the best size to raise.

I think raise to $15-20 and be ready to ship turn if they both call and its a blank. If you only get one caller, bet around 2/3 pot, ship river?