One of the members has written a very good piece on introducting cats
which you can read. You may need to start from scratch and reintroduce them. I've had to do that a couple of times because vet visits can make one of the cats unrecognizable to the others (she smells funny to the rest of them).
Also, there is a homeopathic product called Rescue Remedy that you can put into their water to help calm them down.
Another option is to use the Feliway diffuser and sprays. These are cat pheremones which help calm down a lot of cats. It can be expensive so you may want to look around the site and the internet to find a good price.
Play with them together and provide lots of treats when they are together.
So good luck and let us know how it goes.