I actually read Equal Rites far more recently than I first read the next two “witch books” by Terry Pratchett, so all I knew was that I liked them. Even on rereading Wyrd Sisters and Witches Abroad, the moment I put them down I can’t remember one bit of the story, or which book or which. What I do remember clearly is which witch is which.
Wyrd Sisters is a sort of parody of Macbeth, and Witches Abroad tackles the whole realm of fairy tales with evil queen looking into lots of mirrors at the core. I liked her. I also loved the section where Greebo the tomcat gets turned into a man, with plenty of confused women and hapless soldiers around. And these are the books where I met the three witches of Discworld.
If I might steal a term from Anathem, the three witches exemplify three iconographies about witches. Granny Weatherwax is the old-style cranky old woman that you really want on your side. Nanny Ogg is the midwife and herbalist mistaken for a witch. Magrat is the Wiccan attempting to revive a lost religion. Put them together, and I suppose you can write just about every story about witches you need to write.