Hi
I have been a dog owner for many years but understand that I am by no means an expert.   Ive  always started with a puppy and watched (mostly GRs) grow to adulthood with happy and calm personalities.   Although not trained in too expert a fashion, they have always known the basics ie sit, wait, etc and this has suited me.   My current dogs have long daily off lead walks - leads on just to go to the car.  They come when they are called, they do (mostly) what I want them to.
Three weeks ago, I brought home a 3/4 year old Golden Girl from Dogs Trust.  She came from Ireland, was a stray and had no history.   She was taken by a rescue who spayed her, then given to DT. 
Initially she came home for less than one day as she had a bad bout of Kennel Cough.   My 2.5 year old lab has a low immune system so they thought it best to keep her at DT for the duration of her illness.   Both my other dogs were very ill but thats another story!!
That first night there were no problems with the dogs.   Perhaps Daisy was too ill. She spent 2 weeks back at DT she was in a 'cell' on her own with no view of the grounds and no company apart from me visiting most days for a couple of hours.   She seemed very stressed, clingy, jumpy with the saddest eyes - looking at her eyes, made you want to cry!   She just wanted someone to pet her.
When we brought her home, she started snapping at my two dogs.    My (girl) lab is small-ish and Daisy is quite tall about the same size as my (boy) GR.   She snaps at them when they come towards me when I am petting her but that has improved.  They dont appear to take any notice of her and carry on whatever they are doing.   I say NO, push her out the way. Last week when I called the lab and she didnt come upstairs, I went to see why and found that Daisy was blocking the stairs.   I stopped that by just pushing her away, saying NO, not making a big thing of it.   Today, I called my GR and I saw that she was blocking him from coming up.   Ptreviously she has only 'bullied' my little lab.
In the first couple of weeks, I let her run off lead but last week I found that she was running out of control and not coming when I called and to make matters worse, the lab who had always come when called, was behaving like an absolute brat (!) as she ran to keep up with Daisy.  My GR was sticking close to me and not investigating his surroundings as he usually did.  Something seemed to be happening but I wasnt sure what. I kept her on an extending lead all week. My two dogs were back to their usual selves.   I finally let her off yesterday and her behaviour was improved and the other dogs seemed happier.
Today however, it seemed as though she was running wild again so put her back on for a while and then took her off after a while.   She seems better after a spell on the lead but I cannot fathom out what is going on.
In addition to this, at weekends, I walk with a friend and her 3 dogs (12yo GR, 5yo Rottie, 4yo lab-recently rescued).   Weve walked together for years and our dogs are all friends.   Daisy has started snapping at her dogs now and there have been a couple of tiffs.   She is fine when we are walking but when we stop (for coffee and sausages for the dogs) Daisy seems to want to take possession (does this sound like a crazy thing to say?)  of the bench or my bag or anything of mine and snaps at any dog that comes close.   She will allow my friends GR to lie beside me when we stop  but wont allow the other dogs near me or my bag or anything of mine. I attached her lead to the bench yesterday and she was better but wouldnt allow anyone to come near 'her' bench.   She seems to prefer being on the lead.   I feel as though she has been kept in a small enclosure because she likes to be enclosed - wants to sleep under the bed, stay behind a bench, etc but on the other hand, when in the garden, she wants to dig under the fence as though to escape.   I understand that she does have issues, that life hasnt been easy for her and she is very insecure.   It feels as though the nicer I make her life, the more she fights against it. I am not sure what to do or whether I am the person who can make her feel better.   She is a lovely dog with a sweet pretty face and a lovely soft pale coat.   She has improved LOADS since Ive taken her but has such a way to go and I wonder what effect this would have on my other dogs?
Please help me.   Im not sure how to make things better for her or even if I can.
Thanks
Michelle

Three weeks is such a short time and she still sounds very stressed to me. I would crate her to give your other dogs a break and because she seems to want a place to escape to (and so that she can see them interacting with you without being able to intervene) and I would also be walking her separately to teach her what you want on walks. You seem to be expecting her to fall into your existing pack's behaviour without putting in the work so that she actually knows what you want of her. Have you got a good dog training club where you can take her so she can gradually learn to obey you with other dogs around and, imprtantly, have some one-to-one time with you. She may have been an only dog before so may not be used to sharing your attention, could also be that she will naturally end up being higher up in the pecking order than your other two dogs because of her personality but you are right to stop what you see as potential bullying. Until you are happy with her behaviour in general I wouldn't be walking her with your friend's dogs, it's too much for her at this stage.