Alex Walter

For me a fold. He bets into 3 players after cold/calling out of SB which means still strength. So I can't see you look good vs his range what consists two pair, sets, pairs plus FD, high FDs and may be wraps.

Even if he over plays weaker hands you still have to be afraid of 2 players behind you who could have you crushed badly with pairs plus better FDs.

What you can do is observe the other 2 opponents and try to get reads. If they look not interested I might call only. Getting it in vs his donk leading range seems way to light to me.

If you think you have no fold equity I like check/call or check/raise if the bet is of a loose and aggressive player. You shouldn't mind getting it in vs a loose player because you still look decent vs his overall range.

If you decide to bet I like to bet big. 3/4 to Pot to get folds of weaker hands with lots of equity vs you like Kxxx, OEs or weak wraps.
If you get called and get a bad turn, check/fold is fine except with if turn is a T obv. Sucks but getting it in vs two pair or a straight with a FD on turn is not profitable with a SPR of 2.

I defiantly flat his 3b in this spot to keep the fish in. If you decide to 4b I commit myself on flop and bet around 3/4 to Pot.
At first you are vs his 3betting range still favorite even if it consists lots of KQxx, two pair and some sets. 2nd you might get two pair and one pair hands with back up to fold and make him give up his equity when you hold just bottom pair.