rats eating their young

I have two rat females being housed with one rat male. About two weeks ago, one of the females gave birth to five babies. Three were cannibalized. My wife and I had been observing them as often as possible. From the last time we checked to see if the obvious larger female had babies, was only a matter of a few hours. 3 hours prior, they were sleeping. Three hours later, 3 dead babies. We separated the mother so she was alone, and 9 days later, those two died. Now, we put the trio back together. Now, the other female gave birth and three were cannibalized.
We do not know which rat is eating the babies. We bought them all together as they were just weaned. We have had them for just over 6 weeks. What is our problem?
Other info. We had them in a 20 gallon aquarium. Plenty of water, plenty of food.
Any info would be great. Thanks

It can be due to various reason

First remember that rats are scavenger and eat their dead now did the baby died and were eaten or were they killed, hard to tell.

Here are a few possible causes.

Mother not producing enough milk.

Mother and other females fighting over the babies causing severe injuries.

Inadequate or insufficient food or water source.

Bad mother.

Stress rare but it happens.

etc

What Deborah said.

It could more than likely have been cannibalism because the pup was already dead, it could have been insufficient protein to either sustain the mother or allow her to create milk, she could've just been a really bad mother, it could've even been that the other mother was "jealous" and ate the baby from the other rat (it happened to a friend of mine), it could really have been almost anything.

A young or first time mother will sometimes eat their young as well. Was this her\their first litter(s)? I've had young mothers do this and then become very productive with their future litters.