We adopted a stray cat over 3 months ago and my other two cats still won't accept him?

Don't Worry, eventually, they will accept your new cat. Nothing in my long experience with cats has ever helped our cats do anything they weren't ready to do, like accept a new family member. Perhaps you can separate them during the night for a while so you at least have peace then. One thing I do always do - I pet and hold the newcomer then with the same hand, I pet our older ones - I hope the scent is carried with my hand and the message that this new guy is really OK and part of the family now.

I had adopted a cat as well, it took approximately 6 months for the other cat to accept him. Cats are territorial, veterinarians recommend at first keeping the new cat in another room, letting the other cats hiss at him through the door because it is their way of communicating. A vet said that for the first few months that people should keep the cats in different bedrooms for a few hours a day letting them out individually for a few hours a day, then putting the new cat away & doing the same for the others. That way they get used to their smell I guess along with surroundings. Outdoor cats that are brought indoor will spray urine on the walls (particularly near litter boxes) to show that they're marking their territory, even if they have been fixed already. I hope that you adopted the stray as a kitten, or you might have to deal with more hostility among the animals.

Hershey, our 'new' shelter adoptee jumped on our oldest female's tail within 10 minutes of meeting her. This set things in stone for life--even after 15 years Dax would not accept Hershey and avoided her. No fights, just not friends with her at all. Merlin, on the other hand, had a willing wrestling partner in her that he loved to knock to the ground and tussle with.

It can take years, they'll work out the problems between them till they find a standoff they can both abide by. Using Feliway or Bach's Rescue Remedy can bring down the stress levels of both of them, though.

it will take a long time for them to accept him, even years.