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.
In terms of what average thickness of 3 coats may be, I'd stop a little short of truely flat with heaviest grit used & proceed to finer grits . sacrificing some perfection for safety's sake, but can still look way above the average nice paint job doing so with good product.
Of course, be extra careful with edges.
Another option is to sand the clear flat (free of orange peel) and then shoot another couple of coats of clear. This will flow out much better and then your cut and buff will leave plenty of clear to provide proper protection.