Cats

Sox on the left, Nox on the right. This was taken probably a couple of weeks after they first got into the house, and they had started to adjust to their new routines.

Oh boy, this hits home! I've had to deal with this a couple of times, and your story sounds very similar to mine.

We adopted two kittens off the street (bro & sis), then the sis passed away :( When we brought in Neo, another kitten, they were at each other's throats for months...until they started grooming each other and playing together all the time!

Then my mother-in-law moved abroad for work, and we took in Big Chungus, their family cat (of 12 years or so). That was around 6 years ago. They still act just like your's do, hissing and slapping and being unpleasant to each other, but they coexist.

Then we brought in Freya, another older cat...a female...that was tough, since the other three were (neutered, but still expressive) males. All of them ended up getting along with each other after a couple of months, except Chungus who had issues with all of them except Neo.

Then Neo passed away :( They all became more chill after that, but Chungus still acts the same way (although, rarely as they might, they do all sleep against each other).

The point you should take from this ~~wall of text~~ story is that all cats are different, both individually and interpersonally. For the longest time, it may look like they'll never get along and, either they don't, or one day you wake up and they're curled up together on the sofa, all cute-like! Just make sure to have {no. of cats + 1} litter trays and water in at least two separate areas of the house so that they don't stress out if they meet there, don't feed them all in the same place, and give it time!

Here they are not trying to kill each other 😝

Wish you and you feline family the best. Thanks for coming to my TED talk.

update? all three of them spent almost an entire day in the same room (as far apart from each other as they could be) just staring at each other menacingly.

and then Nox got up and moved, and Barney hissed and ran away.

but it's progress!

We live in a rural area where sometimes city folk dump pets. So to say we've had a lot of cats is a bit of an understatement. The gentle introduction is great, but if that doesn't work there are a few other possibilities. Getting them fixed will lower, but not eliminate, the desire to fight and be territorial. Catnip might help. Time might do it. A sympathetic vet might get you some kitty Prozac to calm down the aggressive one(s), allowing them to be together without heightened response. A blind play sometimes works, where they're playing with each other under a door or through a box - ie, not face-to-face.

And after all that, some cats just don't get along.