Our three GSD's attacked and killed our Terrier while they were running in the back yard together. Now the two females are fighting. What can I do?

Dear Sir,

At the risk of being told I'm an idiot, here's the Question.

I have had a "pack" of 3 GSD's (1 male, 2 females)for 3 years. Two years ago we added in a small terrier found on the road with a broken leg. After being in the pack and "accepted" for about 11 months, we came home to find the terrier dead. The neighbor reported witnessing the entire pack running the fence line barking at strangers on the ditch. When they turned at the end of the fence one of the GSD's stepped on the terrier, she yelped and the GSD's attacked her.

I assumed I had made the horrible mistake of adding a smaller/weaker animal to a pack which eventually got excited and targeted the "weak" in their frenzy to get at the strangers. (I have encouraged/promoted aggressive behavior at strangers on the fence line. The pack is my security system)

Bad me. I swore this mistake would never happen again and have never added another dog to the pack, let alone a small one.

Well, now 2 years later, I just got a call from the wife. My youngest, smallest female "had enough" of the dominant female last night and the fight was on. She got the two broke up and injuries were minor. She says the two females are bracing and maintaining eye
contact when around each other now, the young one NOT demonstrating the passive behavior she has in the past. I'm afraid the challenge for pack queen has begun and there will be no stopping them.

Assuming I'm right, I also assume there is no solution but to permanently separate the two or let them fight until one is dead?

Any suggestions?

Will