Why is it so often three witches? That’s the number who band together to fight the forces of supernatural evil (in the form of a supermarket chain arriving in their small Cotswolds town) in Witches of Lychford, my forthcoming novella for Tor.com. That grouping is part of a long tradition.
The three witches of Macbeth are the obvious starting point. Shakespeare may have based those fate-deciding ‘weird sisters’ on the Fates—the Moirai of Greek myth or Parcae in the Roman version—of whom there were also three. (The historical chronicles he takes as his source material make this identification directly, making it clear that the term ‘weird sisters’ is another name for the Fates. It also offers the possibility they might have been fairies, but doesn’t actually call them witches.) The idea of the Fates may have influenced Norse belief in the Norns, also a trio of divine female arbiters of destiny.