(3 syl. in Greek, 2 in Eng.). A triple deity, called Phoebe or the Moon in heaven, Diana on the earth, and Hecate or Proserpine in hell. She is described as having three heads—one of a horse, one of a dog, and one of a lion. Her offerings consisted of dogs, honey, and black lambs. She was sometimes called “Trivia,” because offerings were presented to her at cross-roads. Shakespeare refers to the triple character of this goddess:
And we fairies that do run By the triple Hecate's team.
Midsummer Night's Dream, v. 2.
Hecate, daughter of Perses the Titan, is a very different person to the “Triple Hecate,” who, according to Hesiod, was daughter of Zeus and a benevolent goddess. Hecate, daughter of Perses, was a magician, poisoned her father, raised a temple to Diana in which she immolated strangers, and was mother of Mede'a and Circe She presided over magic and enchantments, taught sorcery and witchcraft. She is represented with a lighted torch and a sword, and is attended by two black dogs.
Shakespeare, in his Macbeth, alludes to both these Hecates. Thus in act ii. 1 he speaks of “pale Hecate,” i.e. the mother of Medea and Circê, goddess of magicians, whom they invoked, and to whom they made offerings.
Now ... [at night] witchcraft celebrates Pale Hecate's offerings.
But in act iii. 2 he speaks of “black Hecate,” meaning night, and says before the night is over and day dawns, there
Shall be done A deed of dreadful note;
i.e. the murder of Duncan.
N.B. Without doubt, sometimes these two Hecates are confounded.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Hecate from Fact Monster: