The Morrigan

The original and best Witch.
Granny was apparently them/a part of them. (Source, a three eyed raven, see prophecy below)
Their symbol is a raven. They are known as the Maiden Mother and Crone. The Moons may be named after them, or they may be named after the Moons, or... (speculation).

Prophecy given to Oksana by a three eyed raven:

Blatherwort's end beneath the sun
But do not think that she is done
You have not seen the end of that harridan
For she is still the Morrigan