Three witches's background

There isn't much said about these particular witches. They discuss doing spells on people that have pissed them off in the past. Apparently they have a witch goddess of sorts, Hecate, who they neglect to invite. Elizabethans were pretty versed on their witches so Shakespeare didn't need to do a history on them. They served a specific purpose for the Bard and that's exactly what he uses them for.