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?