Tertullian (160-240) introduced this word into Christian theology. The word triad is much older. Almost every mythology has a threefold deity. (See Three.)
American Indians. Otkon, Messou, and Atahuata.
Brahmins. Their “tri-murti” is a three-headed deity, representing Brahma (as creator), Vishnu (as preserver), and Siva (as destroyer).
Cells. Hu, Ceridwen, and Craiwy. Cherusci, A three-headed god called Triglat. Chinese have the trible goddess Pussa. Druids. Taulac, Fan, and Mollac. Egyptians. Osiris, Isis, and Horus. Elousinian Mysteries. Bacchus, Persophone (4 syl.), and Demeter. Goths. Woden, Frigga, and Thor.
Greece (ancient). Zeus (1 syl.), Aphrodite, and Apollo. Icsini of Britain. Got, Ertha, and Issus.
Memcans. Vitzputzli, Tlaloc, and Tezcatlipoca. Perucians. Apomti, Chureonti, and Chemoth. Persians (ancient). Their “Triplasian deity” was Oromasdes, Mithras, and Arimanes. Phoenicious. Astaroth, Mileom, and Chemoth.
Romans (ancient). Jupiter (divine power), Minerva (divine Logos or wisdom), and Juno (called “amor et delicium Jovis”). —Vossins: De Theologia Gentil, viii. 12. Their three chief deities were Jupiter, Neptnne, and Pluto.
Scandinavians. Odin (who gave the breath of life), Hænir (who gave sense and motion), and Lodur (who gave blood, colour, speech, sight, an hearing).
Tyrians. Belus, Venus, and Tamuz, etc.
Orpheus (2 syl.). His triad was Phanes, Uranos, and Kronos.
Plato. His triad was To Agathon (Goodness). Nous or Eternal Wisdom (architect of the World) (see Proverbs iii. 19), and Psyche (the mundane soul).
Pythagoras. His triad was the Monad or Unity, Nous or Wisdom, and Psyche
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894