PB aggression towards another PB in family

[hisbridget]

I have a 2 1/2 year old pit bull female. I also have a 1 year old male. They have been together since Thanksgiving. They play fairly well together. Occasionally, the female would get after male but it sounded way worse than it was and I could always break it up by the sound of my voice or the use of the water bottle. About a month ago, a 15 pound male dog was added to the pack. (We also have a Newfoundland). The female is second while the male is third in the pack out of four. When Thor came to us he was very dog aggressive. I have worked with him at length and he can now be walked on a leash with appropriate behavior. He had never shown dog aggression towards the other dogs. However, within the past two weeks, the female has been tearing into the male. The male will always back down once we break the fight up while the female is ready to kill him. They are both great dogs but I am extremely concerned about the aggressiveness that the female is showing. Before, if a dog came up to her no matter how big or small she would submit to that dog. My question is can they (more specifically can female) be retrained to get along with Thor? And if so, how? Is the aggression something that they will always have towards each other. Please note that neither Pit Bull shows aggression to the other two dogs. Thank you.