For me, it depends on three factors, how aggressive the opponent is, how big the three bet is, and whether or not I am in or out of position. 

If the opponent is a nit, I would fold, as you're either way behind or racing, which can only be bad for our long term profit. If the opponent is a maniac/lag, I would just call and see what the flop brings, if the opponent is a TAG, it would either be a choice between calling or folding (by 4 betting we turn our mid pair into a bluff, which we simply don't need to do). 

How I choose between calling or folding would depend on the size of the three bet. If I am getting pot odds of 3 to 1 or more I would call, and hope to set mine. We probably don't have the correct odds to set mine, but poker is a 3 street game, and there are some flops which will be more favourable to our perceived range than it will to our opponent's perceived range. For instance, on a flop like AKx, there is no way we could continue against a bet, but on a board like J9x, against a thinking player, they are going to find it very difficult to fire more than one street with a hand like AQ, as it hits our range hugely. Therefore I would call a flop bet and take it away on later streets (or check down for showdown equity, depending on the action).

Of course, all of these factors depend on whether we are in or out of position. If we are in position its fine to call a 3 bet, as we can do so much to our opponent when we are in position. However, I don't believe that calling a 3 bet with a medium pocket pair out of position can ever be profitable, no matter who the opponent is, unless we are getting some insane pot odds, so it would be a choice between 4 betting or folding. As I said before, I don't like turning medium pocket pairs into bluffs, so it would be a fold practically every time. This may be exploitable to some opponents, but frankly, its such an infrequent spot, and your opponent can have no way of knowing exactly what hand you hold preflop so it wouldn't bother me, let them exploit your pocket pairs in that spot all they like!

I just posted a very related hand.

I've been thinking that it's very difficult to play well postflop i these situations. In particular, how do we play aggressively? If we are 4betting AA-QQ,AK, then we have a lack of value hands on most flops vs. villain's range which contains those hands.

On dry boards with one high card we should be able to bluff catch easily enough (could prob even call a street with those nightmarish middle pairs). 

But on low and/or coordinated boards, surely we have to find a way to play our range aggressively(or else we have no FE with our draws and villain can check behind turn/riv when we hit). Possible options seem to be (when villain is barrelling):

1) check raise the flop (allows villain to shove flop with FE while realizing all the equity of his hand)

2) donk-3bet the flop (realizing the equity of all of the range with FE)

3) check-raise shove the turn (realizing equity of all of the range with FE)

Whatever we do, we're faced with the problem of having few hands better that beat villains overpairs. But if we 3) check-shove the turn, then we at least get him to commit half his stack with some bluffs and we realize the (albeit diminished) equity of some draws.

And maybe we should call the 3bet pf with some combos of AA

(Disclaimer: these are little more than the ramblings of a slightly bewildered man)

P.S: FWIW snowie loves calling 3bets oop with middle pairs

Against somebody who is 3betting in position as often as he is, I would start adding some stuff like 88+, JTs+ into my 4bet bluffing hands for balance purposes.  You have some equity with these hands post flop, so they're not pure bluffs, and I imagine you will see him folding to 4bets a lot considering he is 3betting such a wide range in position.   That is my initial thought though, I'm still a newb.  

Imho it really depends on his 5b%  and fold to 4b%. In general I would be 4b-ing a lot here with many AQs/AQo, AJs, ATs, not always but frequently. With JJ I would 4b as well and call AI depending on his 5b/F4b stats. Other holdings I would just fold. I don't like playing OOP especially when calling 3b.

These hands will be difficult to play for anyone. I'd fold up to 88 unless Villian is a maniac.
If he truly 4-bets 19% vs EP, you should 4-bet/call here. That's an insane stat.

Even if you lose a bit when you get it in, it will be less than the amount you lose by playing this hand OOP, and it will be less by folding.

didnt see it was 19% 3bet range. Villain needs to defend at least 8% of all hands vs. a 4bet then or else we can 4bet bluff any two.
 And you can prob just shove 88...if 2.5k hands is enough for that stat.

Someone 3betting IP 6% to 12% seems more difficult

Why shove 88 if you can also 4bet/call? Giving him the extra opportunity to jam with like Ax.

Why do you guys want to 4bet/call with 88? We dont't know anything about his 5betshovingrange here!? I think we can easily figure out how to play against him. If OP can give us more info gor example; -his foldvs4bet

-his 5betrange

-his pstflop 3betpot cbet stats

When we know this, we can perfectly adjust

In that case, we can better fold the pairs and 4bet blockers right?

I dont agee, villain 3bets 19% which is very loose,JJ-99 is way ahead of this range. I never fold those pairs here. You can defend pretty wide here and deside postflop plays on his postflop stats, 

How much does he cbet/barrel/cbetfopd etc 

Yes, but we have stats here!? 2,5k hands!? 

BTW i think you over considering stats,isnt bad at all sometimes close HUD and play without him and make much bigger time for reads ,making notes...

GTO is awesome tool,but GTO is good in situation when you and villain play almost GTO tactic ,but how often it is?

snonk: and one more think ,if you dont have 1k hand and more you make zero adjustment?