The Triple Goddess, specifically Hekate, first appears in antiquity around the 8th century BCE (the period between 800 BCE and 701 BCE), in "Hesiod’s Theogony as a singular Great Goddess of the three realms: heaven, sea, and earth." However, it was not until the 5th century BCE (the period between 500 BCE through 401 BCE) that Hekate appears in triple form, as a Triple Goddess.
"Many Pagans will already be aware the Triple Goddess was the creation of Robert Graves. However, our knowledge of the ancient origins upon which Graves drew was incomplete, until now." [John Halstead, The Secret History of the Triple Goddess, Part 3]
In Europe, during the 8th century BCE (when Hekate first appears), patriarchy had long been well established in Europe. Males wrote of and described their version of the Divine Feminine. During this period, the Divine Feminine was defined by men, for women.
Patriarchal elements were unknown in Europe west of the Black Sea before 4400 BCE. Matrilineal Old Europe spanned the period from 7000 BCE to around 1700 BCE. This earlier, more ancient, period in European prehistory is the age when matrilineal Europe (which included the Ancient Atlantic Megalithic Culture of my ancestral genetic mothers) flourished. This is the time when the Divine Feminine was not defined by men, but by women, for women.