it seems as if you are now being excluded the same way you excluded Danielle and that other girl in 1st and 2nd years. this is the kind of thing that happens when you live in a house of bitchy immature girls. although you say that you've seen the error of your ways and you're sorry for it, you can hardly blame the other girls for wanting to give you a taste of your own medicine. the same thing happened to our group of friends in year 7 (age 12 lol) one girl went through the whole group targeting different people to be made fun of and trying to string everyone along with her until we eventually realised that she was the problem. we were stupid for being strung along and peer-pressured into excluding the other girls. however, its not really the same issue when you're 20 or 21, its extremely childish at that age. if it really is sally that's the problem then maybe you all need to sort it out with her, try and make her see what she's doing wrong.
if you dont want to sit down and talk to them about it, (i understand that could be awkward and cause even more problems in your house) maybe just hang around more with other friends from your course or whatever, maybe do your extra work in the library instead of at home if you usually do.
i had some problems with one of my housemates last year, she stole stuff from my room (clothes and make up) and i confronted her about it, she denied it and then left the clothes in the spare room without admitting that she'd taken them. she already had a notorious reputation for "borrowing" people's stuff, using them all/breaking them and then denying that she ever borrowed them, so for me that was the last straw. after confronting her, we didnt speak for a while, just tried to avoid being in the living room at the same time etc but i suppose it was ok because we both still spoke to the other 2 girls in our house, and weren't totally alone. after a while we started making small talk and it was fine. i just spent more time with other friends outside of the house and kept up a civil relationship with the other girl for the sake of the atmosphere in the house. you don't have to be best friends with your housemates, as long as you can be civil.