With 3 coats, I try not to get too aggressive. You need to leave at least 2 mils after buffing for longevity. As for how much is left, there are too many variables to answer correctly. It depends how many mils your 3 coats are. It can vary a lot depending on clear used and how it was sprayed. Then it depends how much sanding is done to get it reasonably flat. IMO, you need at least 4 coats of high solid clear to sand something flat and still have enough left for longevity.