well, if i got what you mean right then the formula is pretty much the same, tho im not 100% sure here but just in this case that we dont get additional feedback, let say that he opens 3bb and we are 3 betting with 3x of his open

we have 4,5bb already in the pot we are investing 8bb additionally to raise to 9bb total, 8/12.5=.64 so he need to fold to resteal or fold to 3bet more than a 64% of the time to make this move profitable, that of course meaning that we have 0

equity if he calls, but we never actually have 0 equity preflop...

thus being said, the lower our 3bet or resteal is the lower his fold % needs to be, and vice versa the bigger your resteal is the higher his f3b or fold to resteal need to be in order to profit...

again, im saying im not 100% sure here