Hamster Behavior - fighting

Question
We got three dwarf hamsters at the same time from a pet store and they have been living together in a 20 gallon tank ever since (they were caged together at the pet store too).  Everything was fine until a few nights ago when two of them started fighting with each other.  They go at it pretty bad with one of them ending up with cuts on her.  I took the hurt one out of the cage and the other two seem to get along OK.  When I tried to put her back, the other hamster kept attacking her.  Whey the sudden change?  Do I have to keep them apart?  Will it stress out the lone hamster who has been raised with her two sisters to suddenly be alone? Please help.

Answer
Dear Quinn,
thank you for your question.
Dwarf hamsters live as pairs in the wild, but in captivity they tend to fight sooner or later. Breeding pairs might work, but same sex-groups fight in almost all cases. I wouldn't put the single hamster back, the others won't accept her now. It might well be that the other two will begin to fight, seperate them when you see the first signs to avoid serious injuries.
The fighting occurs because even a big cage is too small for the hamster to avoid each other completely and even in the wild they sometimes look for new partners.
I hope I was of some help to you
Jennifer