Do it slowly & it could take several weeks for them to get used to each other. The best thing to do, is put the new cat in a room by herself with food & water & keep the door closed while your not home. This way, the original cats will have the house to themselves & will be able to smell the new cat under the door. This will prevent any serious fights from breaking out. Once you come home, you can let the new cat out, just watch to make sure there are no serious fights and make sure to put her back in the room when you go to bed. Do this for as long as needed, & even if the cats don't become best friends, they will at least tolerate each other.