We have two dogs that got along fine for two years. We recently got another puppy. Now the two dogs fight viciously. Can you tell us why and what to do?

I have a major problem with one of my dogs and I am hoping that you can help me or point me in the right direction. I have two dogs Miranda(chow/shepherd mix) and Abby (lab/retriever). I recently got a new puppy Axel(German Shepherd). Miranda does not like Axel and it has taken some time for her to adjust to him, which is understandable. What I did not expect was for Miranda to attack Abby. Miranda and Abby have been together for two years and are buddies. Miranda is definitely the dominant dog out of the two. On Tuesday Miranda viscously attacked Abby for no reason. She did not draw blood, but I was surprised that she didn't because the attack was that viscous. I can not let Miranda anywhere near Abby because every time she goes near her she attacks her. Miranda has shown no signs of aggression towards my boyfriend or myself. I have pulled Miranda off Abby the three times that she attacked her (which was probably a stupid thing to do, but I had to do something) and Miranda showed no signs of aggression towards me. She acts aggressive towards other dogs(hair stands on ends, forward leaning position, growls) but the only dog she has attacked is Abby. Miranda can not be anywhere near Abby because she will attack her. Miranda has been to the vet and there is nothing physically wrong with her. She starts classes next week but the situation is getting worse rather than better. I am very frustrated and don't know what to do. I cant possibly see a way to solve this problem. Will this situation ever get better? Please help.

Alysia