There is a lot of information on this site about introducing a new cat - take a look at the many posts in this area!

Basically you need to isolate the new kitty and let her get used to the smells of the other cats through a closed door. Occasionally you need to "trade spaces" and put your cats in the closed area and let the new kitty roam about. After a week or so you can put the new kitty in a carrier and let the others sniff around her a bit. However, don't put your cats into a carrier together under these circumstances.

When they stop hissing at each other under these situations you can allow brief supervised periods together. Feeding them far apart but in the same room is a good start. You can gradually increase the time together, but they should not be unsupervised for at least a month or two. This takes time.

You might want to get a Comfort Zone Plug-in with Feliway to help ease the stress. Also rubbing Bach's Rescue Remedy in the eartips is another way to calm them down. Your vet can recommend further medications if things don't go well.

Here is a site you might want to check out - scroll down to "introducing new pets" for a good article on the subject:
Feline Behavior

Good luck!
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Smile, it will make them wonder what you are up to!

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