seems standard. My range would be KK+ probably

If he really is a good reg then this is a fold. Some weak regs do this with JJ and AK.

This should be a fine fold, you dont have to give it much thought and let it bother you, if you get it in you are always behind.If MP decides to call he gonna do it with QQ+, Cutoff already has a very strong range cause he 3bet an early position open, so he gonna have JJ+/AK pretty often too. and the player that is threating you directly, unless he is doing something extremely bad and being on tilt for making a move like this, he gonna have something like TT+/AK in the best scenario for you.You can only call this only if you saw him doing that with any two cards,or extremely wide(tilt), that is highly unlikely at 50NL anyway.