The belief that the various Goddesses (such as Venus, Aphrodite, Inanna, Astarte, and Ishtar) are parts of the Great Goddess (a single Goddess) is prevalent in modern Paganism. The Great Goddess is often understood as the Triple Goddess, who is the Maiden, Mother and Crone. Examples of the Triple Goddess for modern Pagans are Diana, Isis and Kali or Kore, Persephone and Hecate. (Note 1)
The Triple Goddess could be regarded as a Trinity of the Divine Feminine. The Maiden, the waxing crescent moon, is the young woman of new beginnings. The Mother, the full moon, is the mature woman of fertility. The Crone, the waning crescent moon, is the elderly woman of wisdom. Modern Goddess religions often add a fourth – the Dark Mother, the new moon, who is the Shadow of the Mother.
The concept of the Triple Goddess was introduced to modern feminists through Robert Graves’ work, “The White Goddess: A Historical Grammar of Poetic Myth” (1946). Graves, a poet and Greek scholar, theorized that there was an archetypical triad of Goddesses in European polytheism. In the 1970s and 1980s, Starhawk in “The Spiral Dance” and Margot Adler in “Drawing Down the Moon” fleshed out his ideas to be “the Maiden, Mother and Crone.”
The concept of the Triple Goddess did not originate with Graves. It came earlier with Jane Ellen Harrison of the Cambridge Ritualists (Note 2). This classical scholar argued that the Greek religion had triple Goddesses who combined he phases of the moon with the cycles of a woman’s life. Harrison based her ideas on the theories of Sir Arthur Evans, the noted British archaeologist. While excavating Crete, Sir Evans decided that the Minoans (ancient peoples of that island) worshipped a Double Goddess (Virgin and Mother). (This is reminiscent of the Christian Virgin Mary.)
The Triple Goddess is not a part of traditional Polytheistic religions. There are groupings of Goddesses such as the Norse Norns (the Fates). There are Goddesses such as the Morrigan (with Babd and Macha) who are tripartite. However, none of them are tied with all of the cycles of the moon with the cycles of a woman’s life. Many Goddesses do have aspects of the Maiden, the Mother, or the Crone, but not all three.
Note 1. Diana is Roman, Isis: Egyptian, Kali: Hindi, Kore and Persephone: Greek, and Hecate: Greek and Roman.
Note 2. The Cambridge Ritualists, of the late 19th Century and 20th Century, theorized that myths are echoes of rituals, which are a form of magic to alter nature.