Sounds like two of the three pairs of diodes are gone. An independent alternator rebuilder can easily swap the diode pac on one of these, or do a total rebuild, usually for less than replacement.
Batteries and Alternators are interlinked - one goes bad, it overworks the other leading them both to premature death.
It's a good idea for owners to regularly check the health of their batteries and alternators before a problem occurs, instead of getting to autozone once an issue arises.
This is made pretty easy with some of the testers out there. One I like is the
Actron-CP7677-Automotive-TroubleShooter-Multimeter . In addition to all the functions of a normal DMM, it also includes a tach and can test for short cicuits, bulbs, fuses, relays,battery load, alternator load, MAF and TPS sensors and more - it includes a pretty comprehensive manual to perform all the functions. You can even use it around the house. It has 18" leads (incl. aligator clips), a 2" LCD screen that even tells you where to plug in the leads, independent ON/OFF, a Hold button, flip-out stand and shock holster - a lot of tool for the money.
Cheers!