Noun: "(mythology) a female triune deity, being a single goddess with three distinct aspects/manifestations, or three separate goddesses who form a unit"
1998 — Mike Dixon-Kennedy, Encyclopedia of Greco-Roman Mythology, ABC-CLIO, Inc. (1998), →ISBN, page 146:
A mysterious deity, depicted as a triple goddess with three bodies and three heads, she [Hecate] was honored in Heaven, on Earth, and in the Underworld and was held in esteem and awe by all the Olympian deities, including Zeus.
2007 — Evans Lansing Smith & Nathan Robert Brown, The Complete Idiot's Guide to World Mythology, Alpha Books (2007), →ISBN, unnumbered page:
Morrigan is a single goddess. However, she is also portrayed as a triple goddess at times, made up of three separate entities.
2009 — Jennifer Emick, The Everything Celtic Wisdom Book: Find Inspiration Through Ancient Traditions, Rituals, and Spirituality, Adams Media (2009), →ISBN, page 81:
The Irish Brighid was a triple goddess, the name belonging to not one but three daughters of Dagda, each of whom had a separate specialty.
2010 — Gienna Matson and Jeremy Roberts, Celtic Mythology A to Z, Chelsea House (2010), →ISBN, page 22:
The two minor sisters are probably simply other aspects of the dominant Brigit. Taken together, the Three women form a triple goddess.
2010 — Michelle Skye, Goddess Aloud!: Transforming Your World Through Rituals & Mantras, Llewellyn Publications (2010), →ISBN, page 45:
Eriu is yet another example of an Irish triple goddess. With her sisters, Banba and Fotla, she creates a triad reminiscent of Brigid and Morrighan.
2012 — Sharon Paice MacLeod, Celtic Myth and Religion: A Study of Traditional Belief, with Newly Translated Prayers, Poems and Songs, McFarland & Company, Inc., →ISBN, page 51:
The goddess Brig was a triple goddess of poetry, smithcraft and healing, along with her two sisters of the same name.