merging my new cat with 2 old ones

Help! 2 months ago I brought a 10 month old stray (Bikkhu) into my house with my 2 older cats (Amory, age 9 and Allie, aged 11). All are girls. I had the new one neutered. It has been a nightmare since she came in to the house. The old ones hate the new one and the new one constantly attacked the old ones until I finally separated them. They can now see each other through a glass door but cannot get at each other. Its been a month now and there is still much hissing and growling on both sides.
In a calm moment I decided to open the door and see what would happen and Bikkhu immediately went after both Allie and Amory.I had to separate them again.
Any advice on how I can get these girls to get along? I'm very tired.
Namani

First of all...Bravo to you for providing a home to this stray cat.

Please click here to read a thread about this very problem.

When you are ready to do reintroductions of the cats, please click here for detailed instructions on how to successfully bring a new cat into the home.

Good luck!

Also if there is some way to provide a screened opening between the two cats instead of the glass one, that will help a lot.

To add to Hissy's thoughts.....I have 3 mesh baby gates that I stack on top of each other in a doorway. They can see, smell and touch each other without too much direct contact.

Thanks everyone for your responses. I originally had a screen door in front of the office wehere I had the new cat. I had taken it down cause I thought we were through the worst part. It had to be taped up in several places to keep it up. Anyway I think I might be ready to try again. I had gotten some bad advice, like just let them fight it out and work it out htmeselves. That resulted in my older cats being relegated to the bedroom, much like the victims of the Maine Coon Cat that was mentioned earlier.
NOw that my older ones are seeing that I am protecting them they have gotten bolder. I will try this again .
I did get somethign called "Feliway" recommended by my vet, but that isn't really doing much. Also was recommended to keep the new one actively at play to wear her out a little. Will try to slowly re-introduce them.
Namani

I also had 2 cats (they were 4 years old) I found a stray on the highway in a terrible winter snow storm. I rescued him, and after 8 days at the vet he came home with me. Charlie and Gizmo hissed and growled the second he came in the door. All Lucky wanted to do was play with the other 2 cats. When I was not at home,I kept Lucky separate from the other 2 . When I was home I kept them all together and watched them very carefully. Charlie eventually became ok with Lucky. Gizmo still hisses and swats at him, and it has been almost 3 years now. They do tolerate each other, but will never be best friends. At least now I can leave and not have to lock anyone up. I think if you are patient, it will work out or at least be tolerable.
It bothered me so much that they all didn't get along,and sometimes still does, but I know that Lucky is happy, and so are Gizmo and Charlie, so that's what matters.

Good Luck !!