Ahem. Equal Rites was written in 1987. Wyrd Sisters was written in 1988.
Equal Rites, in fact, features only a proto-Granny Weatherwax; her character and the way witching works changes considerably in the later books. (One of Pratchett's strengths, I think, is that he's willing to discard continuity in favour of a good book, although he did fall into the Canon Welding trap in the very later books.)
Biting the hand that feeds IT
