Make sure each cat has it's own litter box. try to keep a screened door between them for awhile, maybe you know someone, a friend or relative who is a bit of a carpenter, who could salvage and hang a screened door between two rooms, so they can sniff each other and see each other yet not come to blows. If you have alternate routes into the room that needs a screen in it, you could , yourself, get a roll of screen and staple it across the door all the way, maybe toenail in a piece of wood to staple it across the bottom. They really just want to get to know each other , but cats are usually not as social as dogs, so they start with the hissing and fighting. This is just a small change to your house, and definitely reversible, but i feel sure you will soon see a cat on each side of the screen, looking fairly comfortable and eyeballing "the other side". If you give this enough time, the cats will know each other and will be eager to play , but give it enough time so they don't have to rush, and neither do you. Try to make sure all three of them are spayed and neutered, and safe when recovering, from the other cats. Give each side it's own food and water until they merge. I think it will be a beautiful friendship. (also make sure everyone is healthy and up to date on vaccinations and worming) Best of Luck to your warm hearted self. A little patience at the beginning will be worth your larger and more dynamic cat family.