Hi Xargle, I have been through exactly this about a year ago. We adopted a cat that the owners no longer wanted. The new cat had 'issues' and the reason we adopted him was because one of my other cats had recently been run over and my Burmese was pining for a friend. So i ended up with my Burmese, the adopted cat and a new kitten that nobody else wanted. With my cats it was horrific at first, i couldnt let the Burmese in with us if we had the adopted cat out of his room (we had to keep him confined for the first few months because of the 'issues'!). The cat fights were endless, the kitten didn't know what was going on, the house was nearly destroyed and from the amount of wounds i had on myself you would have thought i had some tiger in the house instead.
I tried everything, every piece of advise i found was followed to the letter. I nearly gave up, but then we went on holiday and the cats had to go to the cattery and have pens next to each other. When we got back peace was restored - they eat together, even slept together a few months later. They still have their occasional quibbles, but apart from that they get on fine. The only piece of advice i can really offer is to just keep trying and be patient - very patient. The cats will find their own level and heirarchy, and things will go back to normal - ish. Your old cats will come back when they know that the new cat isnt an enemy and once it gets colder at night! Just keep giving them all love and attention, and occasionally their favorite foods.
Hope you have a harmonious three cat home soon.