I am more looking at specific spots.
For example: I open for 3x and get three bet from the button. My line?
or
I 3-bet from the BB and get 4-bet. My line?
anyone? oh and I am playing 5NL
Do you have Poker Tracker?
Impossible to answer this question without knowing villain's 3 bet stats and effective stack sizes. In the following scenarios let's just assume both you and villain start the hand with 100bb stacks.
If a player's 3bet pct is under 3% his range is almost certainly QQ+ AKs AK. Your
equity against that range is 38%. If he 3bets you to say 12bb he has 88bb remaining in his stack. Now let's say you shove 97bb here every time and he folds QQ and AKo but calls with AA KK AKs. Since you have one A & one K, he has 3 ways to make AA, 3 ways to make KK, 3 ways to make AKs and 6 ways to make AKo and 6 ways to make QQ. That's a total of 21 combinations and he folds 12 or 57%. So 57% of the time you shove and win 12bb. Now 43% of the time you shove and he calls. Well 25% of the time you win 105bb or 26bb and 75% of the time you lose 96bb or roughly 72bb. So when he calls you lose 46bb. So the total EV is 12bb*.57 (approx 7bb) when he folds minus- the 46bb when he calls or -39bb.
On the other hand if villain has a 3bet range of 6% his range is probably 88+ AJs+ AQo KQs. Your equity against that range is almost exactly 50%. Now let's revise the same scenario, he'll fold everything but AA KK AKs.
His total combination of hands is 48 and he folds all but 9. So 81.25% of the time you win 12bb or roughly 10bb and 18.75% of the time you lose 46bb (roughly 9bb)so your EV is 1bb.
You need to know your villain's stats. When facing an unknown you have a make an estimated guess of his 3bet range based on observation.