teenage literature book about three witches based in glastonbury.

[watches]

I had a book when i was younger, which I am sure was part of a trilogy with three sisters who were witches (potentially named the wyrd sisters but NOT terry pratchett), one who had a deformed eye, it was based around an event on glastonbury tor and. A main character is a young girl who has a talking raven.... does anoyone know what this book is?
thanks