H and I have 2 cats of our own and we ended up having to keep my ILs cat as well. Up until about a couple weeks ago, our cats were separated from the third one. (We weren't supposed to be keeping him so we didn't want all of them to get attached to each other).
We ended up putting them all together starting with short periods of time until they were all comfortable being in the same room. They seem to be okay now, a little hissing here and there but for the most part okay. H wants to move JR (new cat) back into our bedroom once we're done renovating. I don't think that's necessary. He thinks that they will just never get a long and I think H coddles JR because he was his childhood pet.
Is about three weeks to much time for them to still be hissing occasionally? Should we just separate them again?
Re: 3 Cats
I agree. They are probably trying to figure out the new pecking order with the 3rd cat in the mix. When we introduced 2 adult males to our pack of 2 females and 1 male, it took some time for everyone to figure out their space (this after several weeks of slow introductions). Then our oldest cats turned 3 and I got pregnant, so there was some unrest while they tried to figure out who was king now. I still don't think it's quite settled.
Try some feliway or rescue remedy if you think one or more of them is stressed out. We have had great luck with both products. We use RR daily on our two girls because they are very anxious with DS around. We plan to use it on one of our males because he is now freaked out by visitors for some reason. He never used to be like this!
Also, check out the book Cat vs. Cat. It has been a huge help to us!
Finally, some cats may never be friendly, but they can learn to tolerate each other with time. I had 2 cats growing up and they were never buddies, but they did tolerate each other.
5 cats. 1 baby.