Resident Cats and Rescue Cat Don't Get Along?

I have two cats in my house, an 8 month old kitten (not neutered yet, going next week) and a 2 year old cat (spayed). Recently I saved a pregnant cat, 4 years old, from a really bad situation. I brought her home and it seemed like they all got along and then out of nowhere the pregnant cat started humming at the others and they didn't like that too much and now I can't get them to get along.

The 2 year old cat decided she will only intervene if things start getting too heated between the other two, but the second the pregnant cat sees the kitten she instantly starts humming and then the kitten tests his luck by inching closer to her and starting to cry out.

Right now I have them separated by room, but I hate that I can't let my 2 cats roam around my bedroom like they are used to. I have tried the putting the new one in the crate and letting them sniff and see each other but to no avail. I don't want to give the pregnant cat to the humane society because she has a few weeks left before giving birth and they will spay her. Please help me out! I love all my furry little ones and couldn't bear to part with any of them. I am currently trying out the Feliway Diffuser and it only seems to be working on my 2 year old cat. Please please help!

By Amanda