The third witch - Rebecca Reisert

Their life is a desperate struggle for survival: wise-woman Nettle, Mad Helga, little Gillyflower, the girl they took in seven long years ago.
They live by selling potions, begging for food, sometimes robbing the dead of the battlefields that scar the Scottish hills every year. But Gilly remembers a happier life. She has dedicated her very being to destroying the man who destroyed that happiness, even though she has no way to reach him. Until the day he saves her life. Now Gilly knows where he is, who he is. Disguised as a poor servant boy, she insinuates herself into his life, subtly begins to shape it.

Even as Macbeth becomes King of Scotland, Gilly brings her revenge to its triumphant conclusion.