quick preflop spot.

opener is some nitty 16/14 over 90 hands (though most aware players will be playing pretty nitty with the fish and me to your left). dude to his left is 36/14 fish who has folded to 1/1 3bets but i expect he is terrible in 3b pots and calls way too much. he also has donked 5/7 flops.

my question is, is there merit in 3bet/folding under the assumption that EP nit ONLY 4bets like QQ+,AK and probably calls with a few worse hands like 88-TT,AQs, and that we get a ton of value whenever the fish calls?

PokerStars No-Limit Hold'em, $0.50 BB (5 handed) - PokerStars Converter Tool from FlopTurnRiver.com
MP ($77.45)
Hero (Button) ($56.25)
SB ($50)
BB ($54.60)
UTG ($63.05)
Preflop: Hero is Button with J:club:, J:spade:
UTG bets $1.50, MP calls $1.50, Hero raises to $5.25

good? bad? meh? what's better?

I wouldn't include this hand in my 3betting range here, especially if you expect the nit to 4bet AK. I'd assume both raising and flatting are profitable though.

I like the 3-bet here.. it gives UTG a chance to call oop with hands you dominate and if he folds you are building a pot against a fish.

Since UTG is likely never 4-betting worse, you can safely fold if he does.

Flatting here is really bad imo.. if the board is low cards and utg barrels.. now what? In a 3-bet pot you can stack off but in an un reraised pot you are in the dark postflop.

Why can't we just flat and let the fish come along? Post flop should be pretty easy right? Villain will be playing his hand face up BECAUSE of the fish behind him, and the fish will be playing how fish usually play. Yes no?

I much prefer the 3b here. It's a great spot to squeeze for value with a wider range assuming UTG has a high fold to 3b%. I 3b AT+ / KJ+ and TT+ in this spot.

If your assumptions are right this play might be good. The key stat though is UTG's stealing freq- if it's 8%, then you need to flat. If it's 15%, then you can go ahead and 3bet. That's another way of saying that it doesn't just matter if you get accurate info from the 3bet, it matters how often you get the information you are fishing for relative to the price you are laying yourself.

You also have to understand you put yourself at the mercy of various counterstrategies from UTG. If he is thinking enough to slowplay hands like QQ+ in order to keep the fish in, then all you have done is inflated the pot OOP against the range which you "should" have been calling against in the first place. So, you need to be thinking here of two things: UTG's steal freq, and then UTG's frequency of outplaying you by not 4betting nut hands, then you need to take the weighted probability of getting the outcome you described in OP and measure its EV relative to flatting. My guess (and this is a total guess, you know more about nits at .50/1 than I do) is that flatting is going to be better here.