My parents have a golden retriever, which they've had for about 7 years, and a terrier mix, which they've had for about 2 years. The terrier has always ruled the house when it comes to those two dogs because the retriever is so laid back, but I recently moved back in and brought my 5 months old border collie mix with me (we are not sure what she is mixed with). The puppy is always nipping at the other dogs trying to play, and the retreiver will either play with her or just ignore her. The terrier will get irritated and start fighting with her, which was OK when the puppy was smaller because the puppy couldn't hurt her, and the terrier wouldn't hurt the puppy. But now the puppy is bigger (and realizes it!) and will get aggressive with the terrier when she wants to "play". The other day she actually pinned her down by her next show her she was the dominate dog. I don't know what to do to stop this behavior. I worried it is going to get worse and one day the puppy will actually seriously hurt the terrier, who just isn't strong enough to defend herself in a real fight. This morning the puppy had the terrier pinned in the corner! Does anyone have any suggestions to help me get the dogs to get along and stop trying to dominate each other? I was told I have to show that I am the Alpha Dog so that she will respect me and stop trying to show that she is to the other dogs, but I don't even know how to do that.

Its true that the best approach would be to assert yourself as the "Alpha Dog" so that all three of the others respect you as such (All humans in the household should hold his "status" among the animals)

Start by observing your own behavior. When the dogs fight and carry on, do you cringe or get nervous? If so, remember they can sense that, which already gives them more "power" over you.

Another tip: when they are fighting, grab the aggressive dog by its neck. This doesn't need to be "rough" but you can use your hands like a claw to get their attention (it usually doesn't take much force at all--just meant to redirect the dog's focus). Put that dog in a submissive position such as lying on its side. Hold him/her there until they relax and "give up."

This sounds a bit strange, I'm sure, but its no different than dogs do to each other (only with their mouths!).

Good luck!

Be very careful if you try to break up a dogfight. If they have actually started fighting a quick dash of cold water in the nose will make them let go. Then you can get them to "submit", but it will take considerable time with the collie, as they are much further away from the pack than most breeds.

My dog Buddy was being somewhat aggressive to a dog that came over with a friend for a visit. He wasn't trying to fight but was trying to show dominance by getting on the other dog, etc.
I kept right with him, even put him on his leash for awhile, and every time he tried to snap at, or climb on, or show dominance in any way to the other dog, I said no very sharply and pulled him to heel. I walked around freely with him but did not let him show any dominant behavior to this other dog.
After about ten minutes of this he stopped hassling the other dog, and when he started acting more civil to the other dog, I let him know how good he was. Not with food though! That can create more aggression.
Sounds like you have 2 alpha dogs, which sounds more complicated, but I agree, you have to let the dog know that this behavior is not allowed, and you need to be close by to intercede immediately, not just yelling from across the room.
When you just yell 'cut it out' from across the room, I don't think the dogs really understand what you want. I think you have to be really hands on.

Are the dogs all neutered?