Hasty generalization - the etymology of the different words are not related; for example witch derives from the Old English wicca and wicce (male or female sorcerer, respectively), while wizard derives from the Middle English wys (wise, sage), and werewolf from the Middle English wer (man). There's no common root, the fact that they all start with a "W" in contemporary English is coincidence: they could have just as easily started with "v" (as in the Swedish varulf), or gone off of Latin or Greek roots (as, for example, magician).