Rescued kittens (3 females) suddently died, not related

Within a week all stopped eating, became lethargic and died even though they were under vet's care and were given fluids and antibotics.

Neocropsy on the one who died last night revealed a healthy kitten. X-rays showed no abnormalities. The only findings were a brused stomach possibly from vomiting which happened once. No signs of distemper per the vet. Received one distemper vac. from animal control. Tested negative for leukemia and fiv by the vet.

Kitten No. 2 died in a similar manner. Downhill rapidly; forced feeding needed; kitten refused to open mouth at the end. Different vet examined her after her death. Lungs were okay; appeared like she had some jaundice. She had 2 distemper vaccinations while in animal control.

Kitten No. 3 died in a similar manner. Had complete blood work prior to dying. Blood work per vet (3rd different vet) was interpreted as a liver disease, unable to produce enough red blood vessels. All littermates (males) to this particular kitten are wonderful and have been adopted and neutered. Littermate tested negative for leukemia and fiv. Had 2 distemper vaccinations.

Don't know if any of this relates. I rescue kittens in dire situations, most from animal control. If you can shed any light I'd sincerely appreciate it.

Thanks, Becky