I found a dog...what to do?

On Saturday I found a pit bull mix in my front yard. He was covered in muddy water and hungry so I fed him and gave him a bath. Now he is in my spare bedroom. This is one of the BEST dogs I have ever seen. He is so friendly and well mannered and listens so well. Doesn't beg for food, doesn't jump on the furniture, very potty trained, walks well on a leash, walks well without a leash. Not aggressive whatsoever. The issue with this being is that I already have two dogs. This is their territory. I am having to keep my dogs in one bedroom while, for the sake of not knowing his name, I've been calling him Mr Dog, is out getting attention or potty breaks and vice versa. My dogs go out and get attention when he is locked in a room. I tried to introduce them outside in the street where it would seem more neutral but my dogs are NOT having it. I have a pug and a jack russell and they are determined to go after this pit bull! (lol-i guess size doesn't matter in their eyes) I have had to rip my tiny dog away from trying to go after Mr Dog and it really freaks me out thinking about the bad things that could happen if a real fight happened. Now...(sorry I know this is long) I have gone to the shelter and filed a found dog report, had him checked for a chip, I have posted on Craigslist, I have put up signs in the neighborhood, I walk the dog around on a leash hoping someone will claim him. There is no way he cannot belong to anyone...it's obvious by his behavior that he does. The lady at the shelter said I am doing the best thing for the dog by keeping him at my house until his parents are found because he could catch any number of things from there. They are currently having a distemper outbreak but will call me if anyone comes in or calls to report a dog matching his description. My issue is that if I am working so hard to find his owners but it doesn't seem like his owners are trying to find him...how long should I wait before I try to find him a new home? I have gotten one phone call in 2 days and sadly he wasn't the person's dog. As much as I adore him I just can't handle three dogs in one house all fighting to be top dog.