Why are my female rats fighting?

Is the male in the same cage... Or even in the same room? Males smell and the scent can be quite intimidating to some female rats. Move the male away from the females and they may warm up to each other again. If not then perhaps make a sudden noice like SHHH or tap something like the cage when they fight and they may stop after a while if they really hate the noise you make. Buy the male a friend as well, unless he prefers being alone.