Agressive rescuded dog

Question
Yesterday I picked up a dog (not sure of the breed) in a parking lot.  She's very thin and obviously neglected.  We decided to keep the dog considering her condition.

We have two other dogs, a male that we've had for around 8 years and another rescue dog (female) that we've had for six months or so.  The dog that we found yesterday was fine with the two other dogs until tonight.  She'd slept in my bed with our female dog last night and has only shown slight signs of agression around food.

Tonight the new dog jumped on the counter and when taking her down our female dog growled.  As soon as the new dog hit the ground they started fighting.  The new dog bit our female dogs neck and just wouldn't give up.  So, they're seperated now.  I've tried several times to have them in the same room since and they're fine for a while, but the same thing happens again.  

I know this must be some sort of dominance thing, but I'm really concerned for my female dog.  I'm also VERY concerned about what will happen to the new dog if we bring her to the pound, as I'm sure they're not going to let an aggressive dog like her live or go home with a family.  Any suggestions?

Answer
You can stop this by appointing a dominant dog over the other dog. Usually the storngest dog is the dominant one. You do that by feeding the dominant dog first, calling her first, petting her first...
Whenever your dogs bite eachother you have to hold the dog that did the biting and put your hands around her mouth, and close it untill she starts to whine.
If you wish to give up your dog, you can put flyers around the place about the dog. Or you can look for a SPCA center around you,(sometimes the first letter is the name of the city you are in i.e.:FSPCA for Fresno) or you can give it to any other organization concerned with animals.
Good luck...