I have 3 dogs since their individual puppyhood. All are neutered/spayed. One male is 10 yrs. And was dominate until his front leg was amputated. One female is 8-9yrs. She is beta and non-confrontational, but she is loyal to the only the male. The last is 4 yrs. We introduced the 4 yr. old slowly, and since day one she has wanted to be the leader of the pack. She at first was mostly ignored. As she got older and stronger she was humped regularly by the older female. While she is still the smallest, she is the most aggressive. She kills rats in the barn, and tries hard to kill rabbits. Until yesterday, any disagreements as to who is boss was settled with a lot of yelling and showing of teeth. The young one doesn't want to buckle under. She yells back and comes back for more when she is let up. Yesterday, she and the older female really fought and drew blood on both sides. I barely managed to get them apart. The older female stopped and came to me when I called her and I got her out of the fenced yard. Right now the 4 yr. old is tied to the outside of the fence with her own house etc. on a 30 foot lead. I am scared to put them together, and I am looking for advice. I do not wish to surrender her, but I have no place to keep her apart on a permanent basis, and I'm worried about what could happen if I am not home. I have seen both females blocking the pet door and keeping the other female out. They have the basement which is warm in winter and cool in summer. There are also 3 dog houses outside. I have scolded both for blocking the door. This time it instigated a fight. Any ideas on how to work this out?

Dog aggression is something that may never be cured, and you may never be able to have them together. But you can learn the triggers. Have you thought about obedience classes with all dogs for YOU to establish leadership? It's also a great bonding experience for you and the dogs. You need to teach your dom female, that she is not the leader of the pack. You need to establish this role. And simple obedience classes are a great way to get this point across.

When you have them together again, you want both dogs to expect "good
things" to happen when they're in each other's presence. Let them sniff each other, which is
Normal canine greeting behavior. As they do, talk to them in a happy, friendly tone of voice
- never use a threatening tone of voice. Don't allow them to investigate and sniff each other
For a prolonged time, as this may escalate to an aggressive response. After a short time, get
Both dogs' attention, and give each dog a treat in return for obeying a simple command,
Such as "sit" or “stay.” Take the dogs for a walk and let them sniff and investigate each other
At intervals. Continue with the "happy talk," food rewards and simple commands.

One body posture that indicates things are going well is a “playbow.”
One dog will crouch with her front legs on the ground and her hind end in the air. This
Is an invitation to play that usually elicits friendly behavior from the other dog. Watch
Carefully for body postures that indicate an aggressive response, including hair standing up
On the other dog's back, teeth-baring, deep growls, a stiff legged gait or a prolonged stare.
If you see such postures, interrupt the interaction immediately by calmly and positively
Getting each dog interested in something else. For example, both handlers can call their
Dogs to them, have them sit or lie down and reward each with a treat. The dogs will
Become interested in the treats which will prevent the situation from escalating into
Aggression. Try letting the dogs interact again, but this time for a shorter time and/or
At a greater distance from each other.

Here is a great link to look over, any other questions feel free to ask!

Introducing a Dog to Other Pets : The Humane Society of the United States

Excellent link,lots of good suggestions and informative post.