Is it the alternator

I have a 98 montero sport We broke down in the pouring rain the windshield wipers would not work along with the headlights and all the lights on the dash came on. We ended up putting a new battery in and I was able to get it home. We figured it was an old battery and bought a new one to put in it. Ran fine today until I took my daughter for ice cream and it died again. My husband came and put another battery in it and I got it home no problems. got it in the drive and it died again battery was dead. Is the alternator the only thing it could be? Someone please help I've just had this thing for 3 months!