My cat is destroying my fiance's cats. Help!

It's quite normal for your cat to behave this way. She was overwhelmed at first with 2 cats suddenly staying in her territory but when she realized that she can dominate them, she is now the boss.
I don't think she should have been the one being kept out since she is the resident cat. Have you read the stickies on cat introductions? There are several things you can try - scent exchange, rewards for good behavior, rescue remedy (it's mixed in their water although not sure if they are as costly as the room diffusers). It does not matter if they have been together for days. The fact that they are not getting along, you treat it like it's their first day together and begin the slow intros.
Hope I helped.

Do read the stickies in this section, they cover a lot of stuff and in detail. After you understand the basics you'll have better idea where to go next.

And in case anyone tries to convince you. Do not agree to let your cat be declawed for the other cats sake. Cats only cause superficial wounds with their claws, it's the teeth that are serious. If you're worried about your cat scratching the others, though, you can and should keep her claws trimmed. If you're still concerned you can use a product called Soft Paws to cap her claws with.

I wish you luck with this. IMO, girl cats are real pain in the butts. Often they'll not accept other cats very well or if they do at best it's "tolerating" the other cats presence.

Possibly. You have to try it. And definitely get feliway diffusers set up for the apartment.

Your little girl is terrifying the other 2 girls, thats why the other baby went to the bathroom like she did and why they are hiding. (your cat can also be preventing her from using the litter box, cats are very picky about the pee/poo and do not like to share with someone they dont like. A kitty with a stronger personality will prevent the weaker one from using the litter box. This can also make the kitty sick because she is holding herself from being so scared) They can get really sick being under such stress, so you looking for solutions is very wise. (Your girl is stressed and not comfortable in her surroundings either)

I also want to point out, that your kitty is only acting from her instinct. She is not doing anything "wrong" or purposely. She just doesn't trust the other kitties. Likewise, the other kitties are not doing anything wrong, they are just very scared.

You will have to separate them completely and start over.
So maybe you can invest in a portable A/C unit, it's going to take some time, so you are going to have to be patient, but if you are willing, it can be done.

Separating them is really the only way. You have to undo some things that they think about one another. The 2 girls have to learn to trust your girl and not be terrified and your girl is going to have to learn that they are not the enemy and has nothing to fear from them.

When you separate them, in helping them to associate positive things to each one, you will have to use positive reenforcement.
You need to separate them and not do any introductions until all 3 are comfortable and calm in their 2 separate surroundings. You cannot rush things. Kittie have their own timing.

You can also use vanilla extract and place a tiny drop under their chins, so no one can smell another cat to help them relax in their separate places easier. Place a towel under the door separating them, so they cannot see or smell one another.

After they have all settled down and become relaxed, then you can start with scent swapping. Have 2 towels in each separate location. (preferably where they sleep) the next day, as you feed the girls, place the towel from your girl under the other 2 girls food bowl and vice versa. So eating (which is positive for a kitty) is associated with the other ones scent. Do this everyday, often.

They will learn to associate the smell of the other as positive association.
You will have to do this for several days.
Then you can add things like putting one on a towel and brushing them all in the 2 separate rooms with the same brush, towel swapping as you do for the food.
You can start feeding them closer and closer to a door separating them. (dont do this until after a bit because it could set them back if they are not ready so you really want to make sure they have had some time with positive association)

You can feed them treats at a certain time of the day, swapping the towels.

Any of these things you do, if any of them reacts stressful, stop and try again tomorrow. Cats are smart and can learn. You are just helping them to associate positive things to the others scent, reinforcing that every day, several times a day.

If you keep doing the positives then you can get closer and closer to them seeing one another without being stressed, for instance.... a good distance away from each other, but eating, while the separating door is open. When they are finished, shut the door.

Eventually, you can leave it open for longer. Then start using wand toys to play with all of them together. Give treats, food etc.
Just dont force them to interact face to face or even under the door together, until you have done many, many days of the positives.