I personally don't think mysticism can ever be confined to one form of magic. Some of it is the traditional three, but most mystic sects follow their own rules. Lunar and draconic magic, for example, are both forms of mysticism, yet they use radically different rules. Vithelan dream magic and the weird, unique strikes wielded by eastern martial artists are also unique and, though perhaps similar, working under different rules than the big thee. There are many rune cults, spirit cults, and sorcery schools that could certainly be classed as being "mystic", but there are also hundreds of other ways to do magic. Some of them may not even be mysticism, even if they're different from the big three.