x/r with 99 vs preflop raiser 2waypot (only analysis flop moves)

To be honest, I don't agree with your play - on any street:challenge:

You can raise preflop (because you likely have the best hand and probably don't want BB to join, but calling is not a big mistake). But the flop raise can only isolate you versus a better hand, not a worse hand, especially with an A on the board. From there on it is spew.

I dont like 3bet middles pairs OOP with 3 players back.. Although de best option was 3bet preflop as you said... So its fold flop? I dont like call.. What you think?

you might not like it :), but you have a clear 3bet for value preflop
similar situation on the flop, folding would be very weak, you have to call

Middle pairs have the peculiar property of doing very well HU, decently 3-ways, horribly 4-ways. Five-ways+ things look better again, because now you are decidedly mostly set-mining. Six-ways+ (happens in 9-handed games), it is probably okay to to raise with reasonable high middle pairs because now you are hoping someone else makes a worse set along with you, making a huge pot. Few will fold hands they should fold. I have no formal proof of this, but others have said things along these lines, and they make some qualitative sense.