Need advice ASAP

I was fostering 2 groups of kittens, 3 in each group.
The first group, when arrived, seemed alittle sick. The second group was fine..
The next day group 1 was playing, and group 2 started going downhill.
I would focus on saving one, and then the healthy one would go under, so then I would just go back and forth, until they were all sick..
All out of the 3 I mean.
The other 3 have the sniffles, but thats about it.

There is 1 boy and 2 girls..
The boy died earlier last night, and the girl maybe 5 hours after him.
I found the boy, but I was there with the girl when she passed.
She did a flip flop screaming type of thing.

I could have sworn I read somewhere that it had to do with Feluke/FIV or something , where when they die its almost like a fish jumping out of the water.,etc..
Does anyone know about this,??
I know its not poison, as theres nothing in there to get to.
And it seemed somewhat like an URI all week, they were on antibiotics etc..
I syringe fed them all.
I just want to know if it could possibly be a coincidence that the siblings are dying off like that, or if it is a genetic disease , that could have been given to all 3.
I have one remaining, she was twice as big as the rest, but now watching how the others went, she seems like she might go as well..
It was PAINFUL to watch her die like that, although I knew if I took her to the vet they would have PTS, I thought she would go in my arms, etc..gracefully..but not like that, so if it is a genetic thing, maybe it is better to PTS the one remaining girl.
I just would feel guilt if I could have saved her and not known.
2 antibiotics were tried, etc..So I know that I tried, but its hard.
Also, the other group of 3 are in there too, as I designated that the baby room, and they were the first sick.
They dont seem to be ill right now, which is why Im worrying if it can be spread. They might just have been immune to it.
Anyone that could justify how 2 could go within hours??
Thanks ahead

They were all tested before coming from the shelter, but isnt the test sometimes inaccurate when done so early??
They are about 8wks

You need to take the third one to the vet ASAP. This might be something cureable with the correct medication. Since two already have died it is probably contagious. All diseases take a while to develop so if the other group of kittens doesn't show any syptoms now they will probably in a few days. If they allready have the sniffles their immune defense cannot coop with another disease.
Have you called the shelter? They might have a vet.

Well, i have no idea what it could be, but the "fish out of water" thing? no, definitely not a trait of death as a result of FeLV, most cats basically waste away until there is only one humane option.... keep in mind leukemia and aids are not usually the direct cause of death, they are both immuno-suppressant diseases which leaves cats very susceptible to regular, everyday infections. I hope you can save the last kit!

Without knowing the exact cause of the illness, you have to assume there's a chance it's contagious and seperate the sick from the seemingly healthy.

You can get a false negative on fel/fiv tests if they're done too early, but that doesn't help with knowing what's going on with these kittens

Sounds like "DISTEMPER". Don't have any new cats around for awhile!!! They will get it too. We had it on the farm when I was a kid and lost a few that way.