Rick female rats like all female mammals can get reproductive cancers, infections, etc. and there's basically not much you can do as they don't show signs until it's almost too late. Don't feel bad as these things do just happen when you breed any creature, some females will not survive the breeding/birthing experience. Nothing in this situation would as far as I know impact on her offspring being good feeders or the two remaining ones being good pets. She was dying so she was likely not producing much milk anyways, if any at all. Those remaining two are likely well started on dry food and the water bottle anyways. Sorry you lost her, it's crummy to find a nice rat dead like that.