Once you make the 3bet you're way too committed to fold at 33bb stacks.

If he's super nitty you should be 3betting him a lot and probably trying to use a smaller 3bet size, but 99 is usually too strong.

Basically, if he's folding to your 3bets, mix in a lot of non value hands for the fold equity along with strong hands you'll call a shove with and flat call other strong hands you wouldn't want to call a shove with.  That should be a very good counter strategy to that type of player.

I don't think there is any realistic scenario in which you should be folding 99 preflop while 33BB deep in a HUSNG.

Well, against a complete nit who never 4bets with anything except Kings or Aces - but then you'd have to ask yourself whether 3betting 99 is really the right thing against such a guy.

IMO, once you have that read that you need to fold 99 to his 4bet, then you already know that you should better 3bet with 72o against him than with 99.  I would never ever 3bet 99 with the intention of folding it to a shove.  If my opponent really was that nitty that I'd have to fold, then I'd just flat and see a flop with it.

The only situation where I could fold 99 pre is if I'm on the button, some complete nit makes a huge 3bet and I'm 100% sure she's never doing that with AK - but then, I'd simply play 100% of my buttons, steal almost any pot and grind her down pretty quickly.  So this isn't really any realistic example.