There are so many varibles here. I get new dogs here a lot. Some fit in like they were here always, some take awhile to fit in. A new baby is annoying, be sure to only let them be together under supervision at first. That said, it only took a day before Babushka was running full time with the pack. She had no trouble fitting in. The same with Sherman, he was about 4 months old when I found him and within minutes he was playing with everyone while Chumley, 10 months old, took a couple of weeks to be accepted. I guess the best advice is do it slowly at first and read your other two dogs as to how much exposure to the new baby they can stand. When I bring any new dog home home, I sit in something like a computer chair where they can't all jump on me and scare the new dog. After they calm down and start settling down, playing or curling up in their beds, I slowly lower the new dog in my hands so everyone gets a good sniff, about a minute's worth. Then back into my lap. I repeat until my other dogs are bored, about three times, then let the new one go. It works pretty good for me.