The Three-Faced Goddess is a supernatural being worshiped by the Ironteeth Witches. Could be a reference to the Greek goddess Hecate, also called the Three Faced Goddess. Hecate is the goddess of magic and the crossroads.
The Three-Faced Goddess represents the Mother, Maiden, and Crone. The Mother is represented by the Blackbeack Clan, whose High Witch is always a witch in her prime. The Maiden is represented by the Blueblood Clan, whose High Witch is always a youthful witch. The Crone is represented by the Yellow Legs clan, whose High Witch is always an Ancient.
The Three-Faced Goddess is commonly represented by a witch symbol. SPOILERS: This symbol is found in both the Lock, and the Eye of Elena. The Ironteeth witches calls it the Eye of the Goddess, with the Three-Faced Godesses' three forms: the exterior circle representing the Crone, the upper interior circle representing the Maiden, and the lower interior circle representing Mother. The eye in the middle is known as the Eye of the Goddess, in which lies the heart of the Darkness, of the Three-Faced Goddess.