Welcome to Pet Talk! Definitely the place to come for kitty advice.

You should take the new kitty to the vet right away, because he's almost certainly got an upper respiratory infection. You'll want to keep him separated from Maggie and Bailey until he gets over that, so it's good that they're in separate quarters for now.

Because the new apartment will be unfamiliar territory, everybody will likely be nervous at first. Just give all the kitties lots of love and reassurance.

Once the URI is better, you can let new kitty and Maggie and Bailey gradually start interacting. As long as they don't get into a major fight, don't worry about the hissing. Have a water bottle handy in case anybody does get out of hand. Make sure they each have their own food bowls and litter boxes.

Good luck!