I don't think 10 gallons per frog is a rule of the thumb. It would have a lot of space. There is no exact way to know how much space they need. Anyway, now I think about it, 3 frogs fighting each other isn't normal. One is a bully or they have a disease or they are overcrowded.
Sorry for the mastake,
African Dwarf Frogs are very peaceful and the don't fight, baybe you got them mix up for African Clawed Frogs.
Here is a web site that tells you how to tell them apart. Go to http://allaboutfrogs.org/ then click on Frog Info Central,then to Species Caresheets, then to African Dwarf Frog, and click on How to tell the Clawed frog from the Dwarf frog.
I just brought home a pair of African Dwarf Frogs yesterday, and they seem to be having a great time in their 10 gallon tank, with tetras as tank mates. no fighting at all, just swimming and hanging around.