When you took the alternator back to the auto parts place and they tested it were the other 2 alternators no good?
From what I have read the charging "idiot light" bulb must be good and at least 2watts in order to start the initial current to the alternator if it is burned out or the wrong number of watts your alternator will not charge.
Pull out the alternator connector the one with the 3 slots. With the key
off and the (+) probe (1 at a time and the (-) probe grounded) in each of the big slots you should read the same voltage as your battery(2 fat red wires). You should read no voltage in the 1 small slot (this goes to the charging indicator "idiot" light) skinny blue wire.
I have not tried this but when the key is on you should get slightly less than battery voltage from the small slot on the connector when the key is
on. The voltage may show up as slightly less than battery voltage as the charging indicator light will use up some of the voltage.
If you are not getting battery voltage to the 2 big slots on the connector (key off) you have some problem with your wiring.
If you have no voltage comming from the small slot (key on) you indicator light may be burned out or you have a wiring problem.
The last problem can be the alternator is poorly grounded. Some member have taken a jumper cable connected to the (-) battery terminal and grounded it to the aluminum alternator body; if it starts charging after this it means your alternator is not grounded.