Sounds like one of the fish was a carrier of something.. I used to keep fancy fish with a couple of gold fish. they would be fine to keep together.
I once bought a fish from a shop and it had a bug on it.. It started to eat the front of the new fishes face.. My OH culled it but it must of had more than one on it and passed to another. we spotted it.. (my OH said I was obsessed) any way he got the fish out the tank and with tweezers removed it.
You will probably loose the last fish.. IMO I would cull it if the tale is far gone as it will not be able to swim.. if its not too far gone go the pet shop and ask for the correct ointment to treat the fish..
Cold water/ fancies get stressed quite easily and that could also be the cause.
I had two 3' setups with fancy/cold water fish ranging from 2" fish to two 6" Black moor fish..
Lost a whole tank by just introducing one fish.
I just have two red and white Oranda's I am thinking of adding a calico. But will take my time.. as had the other two a while..
Good luck with your fish. It can be quite heart breaking.