Did you separate them for the first few weeks? That often helps ease the process.

Well, he has good reason to be PO'd. There should have been a slow introduction of the new cats. Maybe try starting over by keeping the new cats in a bedroom for awhile and give him time to calm down. Let the cats get to know each other thru closed doors, then after several days let one of the cats out to explore. They shouldn't have had run of the house from the first day........

IMO, it was his house first so he should get priority.

they didnt get locked up because when they came into the house in the kennel He (my cat) was lose in the house. Their mother just let them out. We told her from the start he dont like other cats. She said it would be Ok. 1st thing my cat did was pen both them in a closet . I had to remove him so we could get them out. I put my cat in the bedroom to try to get the other ones out of the closet. We didnt find them . the next morning found both (other cats ) in a cupboard above the fridge. My cat (aku) was right behind me and when that happen the game was on. He chased both cat out of the kitchen . Then it was in a conner . We locked the other cats up in a bedroom. when I open the door he is in there to get them. Last night I went as far as putting my cat into a hashness so that if he should attack I cant get ahold of him so he dont hurt the other cats. He has ripped up a dogs nose. and he has lived with this dog all his life. He was screaming at my husband. We cant touch him or even talk to him. He is pissed off and at this point We need to to play nice just for a few months. I know this is going to be long road. but anything to help with his anger problem.

You need to separate them for a few months.

Is the friend staying for a short period? As you now see, she was wrong in saying she could just let her cats loose in your house.
If she is staying a few months, that's probably the same length of time it'll take for the cats to settle down.
I wouldn't expect your cat to change much during that time so you might as well accept that the other cats have to stay in the bedroom during your friends visit.
I'm not up on holistic supplements but there may be something available that would help all 3 cats. Hopefully someone here has some suggestions.

Good and best advice... Keep them separated. They can sniff each other through a closed door and maybe decide to play nice, but after their abrupt introduction probably not.

I feel sorry for your cat. It's his house and he should not have been invaded like that. It's called transferred aggression, and he will not be easy to deal with while these cats are there. Cats are weird creatures. Two of mine live side by side for weeks and months and then one looks cross-eyed at the other and all hell breaks loose. Then it's back to normal. Cats are crazy like that.

Is this a permanent arrangement? Many "only" cats just hate other cats, period.

Well, you can get Feliway plug ins for the whole house - but even so, adult cats generally cant be expected to be pals right away. They are not like dogs with their friendly tail wagging and butt-sniffing.

Either these cats will never be pals (although they may learn to eventually tolerate each other, kinda sorta) OR... they may become pals, but the introduction was botched by throwing them together like that. In any event you are back at square one needing to keep them separate until... well, whenever they can smell and see the other cats feet under the door without freaking out. Could take days, weeks, months, or never.

I have 1 21 year old who freaks out at other cats, even seeing their paws reaching under the door, 1 mellow guy who gets along with everyone, and 1 dominating a-h who - 3 years later - STILL attacks the 21 year old if he has a chance, if the door between them is not shut securely enough.

Sorry to be a downer - I know its not what you want to hear, but that is the reality of cat culture. Your friend may have been speaking accurately about her own cats when she said it was ok to put them together, but she shouldnt have assumed anything about your cat. Your cat really is not acting at all unusual for a cat by trying to defend his territory. Cats still have a lot of their wildcat instincts, they havent been domesticated as long as dogs have.

In my case we've made the adjustment to keeping them permanently separated and doing sort of a time-share (old female cat gets run of the house at night, young boy cats during the day). It isn't such a big deal now. Figgy (the dominant a-h) has calmed down a bit though he still cant resist going in to the old cat's space and causing trouble if he finds the door open.

Well thats the worst possible scenario. It may come to that, but if you slow down and start over with the gradual introductions, rub all the cats with the same towel to mingle their scents, let them see and smell thru the door, work up to having them see each other in their carrying cases, feed them where they can see each other but not get at each other, etc. - its very ikely they can become pals - EVENTUALLY.

BTW - back in the day when cats were more commonly let outdoors, I think just having all that space made it easier for them to stake out territories and work it out amongst themselves. Nowadays they are expected to share close quarters inside the house and that's more difficult for them and why the careful process of introducing them is necessary.
GOOD LUCK!

Well last night was long night. My cat found away to open the doors. yes He opened the bedroom door. (our bedroom) He has always been locked in our bedroom at night. All my 4 legged kids are . at midnight the cat fight started. Friend had went to town and she didnt close her door. so her cat where lose in the house. I wasnt going to mess with them but at midnight and 3 am after my cat went on the attack and no sleep for this mommy I had had it . This morning I will say after only getting few hours sleep. I am not a happy camper with the cats. even mine. We (husband and I ) had to open our bedroom window to keep aku from opening the door and attacking the other cats. We had a cat fight in the kitchen . Aku as smart as he is opened every cupboard looking for them. ( the other cats) On top of the fridge. there was a fight. everything that was up there was on the floor. the plant that sits on my table ment the floor. nik nakes that use to be on my stands are no more. yes aku make sure the other cats where not in his house. and there was nothing we could do about it . I am over tired and aku is back at it again. He is now makng the dogs lives hell. even the dog he was raised with. My husband told me to let him be and not harshness any more. let him have the house and if there is a cat fight and one of the others happen to die dont worry about it . She was told her cats would not be allowed free range of the house . I think I will start count hours and days till she leaves for texas.

Kitty wiped :(

What a ridiculous, completely unnecessary, and avoidable situation.

I think you need to have a serious talk with your "friend". It sounds like everyone BUT her is dealing with this mess. SHE needs to keep HER cats contained and controlled. Where does she get off leaving "for town" (all night long?) and leaving her 2 interlopers wandering around the house unsupervised?

Since this situation will apparently resolve itself if/when she moves out in a few months (good luck with that), I would not even bother trying to get these poor animals to get along. I would concentrate on reducing the stress on your own cat (and yourself). Either she keeps her cats in her room - 24/7 period! - or they need to go to a kennel. No one can live like this.

My sympathies are entirely with the cats. The humans should know better.

Since your guy is pretty crafty, and it is his turf, you need to make it clear to your friend that her cats will need to be confined to her room(s) until your cat seems willing to interact, or she leaves with them.

I was in a bad spot a decade ago when I had to stay with family for a few months. I was grateful to be able to keep my cats, but they had to be confined to a room- with all the amenities, but I did feel sorry for them. It will work as long as it is for a limited time and your friend really is a good friend. It is appropriate that new cats be cordoned off. Your guy will probably not be real fond of the door which they are behind.

When we introduced our young guy to our old girl, we put him in a room so she could smell him under the door. She was surprisingly eager to make his acquaintance (she was all around odd for a cat), and while she smacked him a couple of times, the introduction was on her terms because she was the resident cat. Let your guy make the decisions, though it sounds like he's already been conditioned to these cats in a negative way.