Car will not hold charge

Recently my car was dead so I replaced the battery.  It worked fine for 3 days.  Then it was dead again.  This time I replaced the alternator.  It worked fine for 3 more days.   Now it is dead again.  What possible reasons could be that my car will not hold a charge besides the battery or alternator?  The car is a 99 Buick Park Avenue V6 Supercharged.  Thanks!