Here's a little story of what we did...
We just got Eli a month or so ago and had to carefully introduce him to the other two, especially Bunny who can be VERY aggressive towards other dogs, outside of our home. We had all of them on leashes, B and Z with muzzles on, and introduced them outside of the house, in neutral territory. I held E and my partner brought one out at a time, and when we could tell that they were ok, we took that one back in and brought out the other and then when they were all ok, we took them for a short walk. For a day or so after the initial meeting, we kept muzzles on B and Z while they were all loose about the house.

Eli did pee once, on Z's bed... I'm not sure if it was a puppy accident or intentional.. but I immediately took him outside and it hasn't happened since. They are all pretty well adjusted now. Z growled for about a week... he didn't like Eli walking over him, or stepping on him -- understandable.

So... Id say just take it slow, watch them while they eat, and eventually they'll all settle down and get along. If there is a lot of snapping or growling, I would suggest a muzzle. Seems to calm the aggressive dog down until they've accepted the new dog. Also, don't baby the new dog too much in front of the other ones.. as the other ones will get jealous and cause more problems..