It may be that the three males started in at the females and they got injured and died or just simply died. Rats will consume dead rats. It's gross to us but to them it's just a free all you can eat sort of buffet. Makes sense from a rat's point of view since for them it's better to consume that protien than have a dead stinky rat laying around. If all your conditions were fine sometimes it's just unhealthy rats that came from a poor supplier though I'd suggest it's likely best, even with feeders, to keep the males and females seperated once they've reached or gotten close to sexual maturity.