But what you should rather be asking why do you 3bet in first place vs him, what's the purpose of the 3bet?
because i'm automatically 3betting AJ/KQ when BB vs BTN...if the opponent is loose,i'm 3betting for value.if the opponent is tight,i'm 3betting to bluff.is this a leak?
Against such a nit most likely I wouldn't be CBetting. He will usually have PPs in his 3bet-calling range there.
if he could call 3bet with all of the pps,why not to cb?