I agree with everyone else here..it requires patience,patience,patience!! We had 2 cats that had never been “best buddies” for their 3 years together, and just added a kitten to the mix 3 months ago…we kept him in a separate room so the other 2 could sniff him out under the door; switched the other cats’ blankets with his, and vice versa, so they could get his scent. We kept them this way for 2 weeks, cuz the kitten had a URI (and is otherwise physically challenged. We’d feed the kitten next to the door on his side, and the other 2 next to the door on their side.
Once Buster’s cold was gone, we opened the door…spitting,hissing, and growling from all parties! We made sure the other 2 cats had their nails trimmed and always supervised..when things got out of hand, we put Buster back in his room (he always instigated trouble anyway, being a kitten!)
Now they basically “co-exist”; there’s no cuddling together, but they do play and roll around and eat together; there’s also pretty much a daily squabble for cat supremacy! But no one hurts the other; at this point, it’s alot of posturing…but it has been over 3 months! (there’s that “patience” thing again!) They all have their favorite spot in the apartment and have pretty much learned to respect each other’s boundaries..so it really will work itself out..don’t get discouraged!