Unfortunately, it is not so unusual to get three bad remanufactured alternators in a row. I've had it happen twice (once on a Benz, once on a VW).

That said, each of the alternators failed for different reasons (bad diode, bad voltage regulator, case screws came undone, etc.). If yours are all failing the same way (bearings), then I'd look to the belt/pulley tension/position (as it sounds like you are).

Sorry that I can't be of more help. Just wanted to share that it may not be your car that is at fault.