First of all, welcome to TCS!
Glad you found us.
First and foremost, the number one "rule" in socializing ferals (and semi-ferals) is patience. I cannot stress this enough.
Secondly, you have rescued an older feral. She knows you and probably trusts you, but she will take longer to come around because of her age.
Last but not least, she is scared out of her wits. Cats, though they can totally love humans, for the most part are very territory-oriented. Unlike dogs, they don't adapt quickly to being anywhere their "human" is. Changing territories is scary and traumatic, and wow did her world just change! She's been outside for six years. Now she's inside. The sights, sounds, smells are all completely different. From her perspective she's boxed in with two strange cats whose territory she's being forced to invade.
So for right now, under your bed is her safe space. This is her territory, and don't be surprised if it stays that way for a while.
Is there a room where she can be kept alone without the other kitties being able to have access? Being moved again will be scary, but if it's an option, in the long run it may help. If that's an option, and if you can, put a screen door on that room. Then she can see out, hear and smell more easily - but she'll be safe to create her own territory and make it hers.
Again - the most important thing when it comes to socializing ferals is patience. They do not operate on a schedule, and any sense of nervousness on your part will delay the process. Just having a sense of urgency will likely cause you to do things that could slow that process down. Forget the clock and the calendar, and things will go ever so much better!
Whether she's in her own room or your room, here are some things that will help her.
A radio is good - tune it to a classical station. It helps calm them. Or, put a small CD player in there with classical CDs. Harp music is best if you can find it.
Routines help kitties a LOT. Get a schedule going. The more regular you can be about the time you spend in whatever room she's in, the better. Go in there at the same time every day to make sure the food dish is full, or to give her a meal of wet food. Go in there at the same time every day to scoop litter. Go in there at the same time every day to give her fresh water. Go in there at the same time every day to just sit and read to her. Cats are creatures of habit and they LOVE routine. This alone will help her feel safe.
She hasn't forgotten who you are. She just doesn't know where she is. She doesn't understand she's safe here. She doesn't understand how much she WILL enjoy being loved. She doesn't know what play is, and she doesn't understand how much she'll love it.
The quickest way to help her come around is IGNORE HER. The more you try to interact with her, the more she thinks you're trying to get something from her or do something to her. Just ignore her every time you go in there for a few weeks, at least.
The more you ignore her, the less of a threat you will seem to her. Again - it's not that she's forgotten who you are, but now you captured her and put her in a scary place that smells like strange cats that attack her. You need to regain her trust.
Don't look her in the eyes - this is experienced as an act of aggression. Best not to look at her at all, but if you do, look over the top of her head. And the more time you can spend in the room ignoring her but doing stuff that has nothing to do with her - reading - reading out loud - singing - doing whatever you can do to just be in there - the better. And as you spend time in there not trying to interact with her at all, the safer she will feel. And eventually she'll get curious. If there are toys in there for her, you may find she starts to play with them while you're asleep. She'll watch you from under the bed. She'll get used to the routines, the sights, the sounds, the smells - and most importantly, she'll begin to feel safe the less you try to interact with her.
When she starts to get curious, don't go for it. Keep ignoring her. Look at her - but not in the eyes - and close yours. Leave them closed for a few seconds. Do it again. It's a really slow blink. I call it "looking at kitty with your eyes closed." This will help build trust and totally communicates that you have no aggressive feelings towards her and you are not a threat.
If she comes out to watch you, do not get excited (outwardly). Just keep reading or do whatever you were doing. Sitting on the floor - at her level, is actually the best thing. You're far less threatening that way. Put one hand down on the floor - palm down, but not stretched out to her. If she comes up to investigate, just leave it there. But the palm down, with cats, is very important.
She needs to understand that she can approach you and you will do nothing. THEN when she's comfortable, you can try reaching your hand out (palm down) - she's learned that that hand won't hurt her.
Then you can try feeding her wet food from a spoon. Engaging her in play with a wand toy.
In the meantime, get Feliway - either the spray bottle, or, if you can afford it, the room diffuser. Feliway mimics the "friendly" scent markers in cats' cheeks, and often helps calm scared cats. You can also try Flower Essences - Rescue Remedy would be a good thing to try. http://www.catfaeries.com
Here's another good article on socializing ferals: http://straypetadvocacy.org/html/soc...feral_cat.html
And as to helping things between her and your existing kitties - again - separating them is really the best thing. If that's not an option, consider purchasing one of those really large multi-level cages for her, putting it in a corner of your room, and mostly covering it with a light blanket. For an hour or more a day, or an hour at a time several times a day, put the other kitties out of that room, close the door, and open her crate door (without looking at her) and go about your business. Again - doing it on a schedule is best, if you can manage that. After an hour, maybe even leave the room for a half hour or so - and once she's learned the routine, she'll know she can come out safely to explore.
The goal is to help her realize that not only are you not a threat despite having turned her whole world upside down, but that she is safe in her new territory, and to encourage her to make the territory hers.
You can also rub your two kitties with a wash cloth and leave treats out for her on them. Rub her with two cloths, and leave treats out for your other kitties on those. This will help them come to associate each other with good things. Again - doing this on a schedule (like once in the morning and once in the evening) will help.
Thank you for giving this kitty a home! With lots of time and patience, she will come to understand it is HER home. And the bond you will form will be like nothing else you've ever experienced.
Of course, we always love updates, and any other questions you have, feel free to ask!