Incase it gets nasty. Then put the girls in the cage, let them settle back in and then add the new girl. Take away any objects that they can hide in, so this moment. Keep an eye on the girls, there will be a lot of squealing, when it gets to much, stick your hand in to break it up. don't remove the new girl unless it gets bad.
I would keep the new rat in a carrier case and let your current girls sniff her out, keep the lid on. Once their attention fades away, take the new girl out and hold her, let the girls come to you. If it goes well, let them free run together. Not in the cage but on neutral ground i.e. a room floor. Leave them like that for about an hr but make sure you're still around.....