First, let's establish some parameters for what makes us pot-committed. Preflop, I like to stick with a 2-1 ratio, since the only way to be definitively more than 2-1 behind your opponent would be to know that his range is almost entirely made up of hands that have you dominated. We definitely know that this is not the case if we are short-stacked preflop.
So, we know that we aren't folding if we are getting 2:1 or better. From there, it can be a math problem dependent on the size of our 3-bet. Excluding dead money from blinds/antes/etc, we will be getting 2:1 any time our 3-bet is for 1/3 of our stack or more. I'd submit then that the smallest stack size you can ever 3-bet/fold is the one where you can make a meaningful 3bet for under 1/3 of your stack. "Meaningful" will depend on your opponent.
Ex. Using a 3bet of 7bb:
Opp bets 3bb, you 3bet to 7bb, and he shoves. If you have 21bb, then the effective pot at this point is 7bb from your raise + 7bb from his call + 14bb from the remaining effective stacksize in the shove = 28bb. Since it's 14bb to call and win 28bb, we must call.
The same ratio will hold for any size of 3bet; if we 3bet to 8bb, we need a stack of over 24bb to fold; 9bb, we need over 27bb; etc.