How do I stop my rats from fighting?

I have two female rats that are a year old, and recently, I got another that is about 5 months old. One of my older rats has accepted the little one, but my other (the dominant rat) will attack her and show signs of aggression, like approaching her sideways and stiffening up her fur. I've introduced them to each other slowly, and I've tried everything I can think of to help them get along, but it doesn't seem to be working very well. Any suggestions?

[ireneo]

How long have you had the young one? If it's only been a couple of weeks, give it more time. I tend to share the bedding between cages for awhile till they become used to each others scent. I've done it with rats and baby squirrels that came from different litters. It may turn out that your one female just won't tolerate a 3rd rat in the group. In that case you can either pair up the small one with the quiet female or keep it on it's own.