I don't think I brought this up in this discussion, but I meant to, and my current signature - Wyrd bið ful āræd* - reminded me of it.
The title - Wyrd Sisters has a meaning that some readers might not have 'got'.
Wyrd is an Anglo-Saxon word meaning 'fate'.
Here's the Wikipedia for Wyrd
In Norse mythology the Fates, or Norns were three female weavers who controlled the destiny of men.
The whole idea of Wyrd Sisters refers to this I think - with our trio playing that role. This is not just a Shakespearean allusion.
It's a very clever title by Terry.
*Fate remains wholly inexorable