Hecate, or Hekate, was an ancient Greco-Roman goddess associated with witches, magic, and death. Over time she came to be represented with three faces, three heads, or three separate bodies, to represent various facets of her personality or function. She also made up one third of the Italic goddess Diana Nemorensis, who was made up of divine huntress, moon goddess, and goddess of the underworld.
Gaiman fuses Hecate with the three major Norns, maidens from Norse mythology who controlled fate, called Urth, Verthandi, and Skald (or Urd/Wyrd/Fate, Verdandi/Present and Skuld/Future), as well as the more commonly known Fates from Greek mythology, called Clotho, Lachesis, and Atropos. The three witches from Shakespeare's Macbeth evoke the same idea.
Gaiman uses several common depictions of the Fates in his characterization, including their being voracious eaters, having interchangeable bodies or personalities, and being represented as three stages of womanhood: the maiden, the mother, and the crone, or a variation on those three.
The Fates also refer to themselves as Morrigan, an Irish triple goddess made up of Badb, Macha, and Nemain, three war goddesses; three avenging sisters in Greek mythology named Tisiphone, Alecto, and Megaera; the three members of the 1960s vocal group The Supremes: Diana Ross, Mary Wilson, and Florence Ballard ("Candy" referring to a replacement member named Cindy Birdsong); and Mordred, Mildred, and Cynthia, the three witches who hosted DC Comics' 1970s horror anthology The Witching Hour. One of the women indicates that Mordred's name was meant to be Morgaine, both names of characters in the legend of King Arthur, but Mordred, the illegitimate prince, was confused with Morgaine, the witch.