You have a failing automatic transmission. The problem is internal to the transmission. IF 3 mechanics independently of one another could not pin point the problem, then it is not low transmission fluid, or just a bad donut. IF this car is in warranty they owe you, if not you are looking at most likely a new tranny. You dont say how old it is, what model it is, or how many miles are on it, or what kind of driving city or road? Have you had the filter changed, is it old enough to have adjustable bands, does it have a vacume valve on it? If it is that old, you could get away with a regular servicing, but I doubt it.