Car keeps going through alternators

I have put 3 alternators in this car in 2 months it will run fine for a couple weeks then the headlight will go dim while driving it then the battery and parkbrake light will come on I’ve changed the battery too I don’t know what else it could be

Do the alternators test bad when you change them? Where are you buying these alternators and have you had your electrical system tested after replacing the alternator." Are they being replaced under warranty by the parts store?