Hecate is one of the favorite Greek deities to try to throw into a ‘Maiden-Mother-Crone’, ‘Triple Goddess’ framework. The idea of a primeval ‘Triple Goddess’ as the original form of much of Indo-European religion, and its overthrow by patriarchal religion, largely originates from Robert Graves’ largely baseless The White Goddess (though he seems to have derived the seed idea of a primeval goddess religion from earlier writers). It has been picked up by Neopagan movements and thus disseminated widely, but it is based on little to no evidence of actual ancient beliefs.
But in fact Hecate is not originally triple in any way, and thus cannot derive from some supposed primeval triple goddess. Hesiod writes about Hecate at length in his Theogony, and nowhere is there any hint of triple nature. In early sculpture and in vase-paintings, Hecate is portrayed as a single female figure.
So where does the ‘triple Hecate’ bit come from? Well, in the Hellenistic and Roman eras, Hecate was commonly portrayed this way. Later sculpture does portray three figures or a triplicate figure. Pausanias (Description of Greece) believed that Alkamenes was the first to portray ‘three images of Hecate attached to one another’ (he is contrasting this with an image attributed to Myron with ‘one face and one body’). She is often given the epithet ‘trimorphis’ (‘three-formed’). However, even this triple Hecate is not so straightforward as is sometimes presented — at times Hecate is described as three-headed indeed, but with animal heads!
The Roman tendency to conflate deities, both local (eg ‘Venus Cloacina’) and foreign (‘Zeus Ammon’, ‘Sulis Minerva’) with even slightly similar aspects with one another began to confuse things further in the Roman era. The Roman loose-equivalent of Hecate was Trivia (“three ways/roads”), goddess of crossroads. Hecate’s identification with Trivia likely contributed to the increased emphasis the triple aspect of Hecate. Seneca (in Medea and Phaedra) gives Trivia or Hecate the attributes of both Artemis and Selene. Statius (in the Thebaid) gives Diana an underworld aspect and the hounds of Hecate. Nonnus (at the end of the classical Roman era, probably somewhere in the neighborhood of 400 AD) speaks of Artemis, Selene and Hecate as the same being.
But all these multiplicities are not original, and so Hecate cannot be a survival of a Graves-style ‘Triple Goddess’.