Everybody fold to the BTN who 3bet (9% BTN vs UTG open) to 21$
9% looks like 77+,AJs+,KJs+,JTs,T9s,98s,87s,AQo+,KQo
Others players fold.
Back to Hero.
1- I can fold (to tight, I'm ahead of his range with 54% equity)
2- If I 4bet, he will always flat my 4bet (this is 2017 poker)
3- If I call, I'm OOP and 75% of the time, the board will bring an overcard and he is very agressive.
flat and you have left out a lot of useful info that you clearly have like sample size on 3b v utg, other 3b stats, and all the postflop stats that you have accumulated which will be more helpful than "he is very aggressive"
Call and play post. Too many hands here where he may see you as having a wide range of opening hands. And even with one over, how often will he hit comparatively? I think it may be a call vs a fair amount of opponents unless he is a total nit.