The resonant symbolism of the number three runs through Celtic tradition from earliest times. The Celts, like other Europeans, attached significance to nearly all frequently used numbers but gave the greatest to three. Triune and tripartite figures appear from the earliest times, while in Wales and Ireland traditional learning was formulated into the Triad. No gloss in a Celtic language explains why this should be so, but commentators apply interpretations that have arisen in other European contexts. Pythagoras cites three as the perfect number, signalling beginning, middle, and end; a tripod or three-legged stool (as at Delphi) is stable and will not rock. Three can represent life: male, female, and progeny; time: past, present, and future; the visible world: sky, earth, and underground; space: before, behind, and right here. Some commentators, especially the influential Georges Dumézil (1898–1986), have suggested analogy with the tripartite division of early European society: farmers, warriors, and clergy. As Joseph Vendryes pointed out (1952) triune figures often have a single dominant personality and two ciphers; perhaps there is really only one person referred to, as can be seen in early Irish dynastic records that cite triplets all with the same name. A prime example in mythic literature is the three sons of Uisnech: Noíse, the lover of Deirdre, has a developed personality but his two brothers, Ardan and Ainnle, are distinguished only by the tones of their voices. At the same time, the reverse of Vendryes's thesis is also true; the three figures of the Mórrígna, Badb, Macha, and the Mórrígan, have much in common and are also easy to distinguish. Further, there are also three Machas.
A full account of Celtic instances of triplism would fill many pages, coming from all periods of Celtic culture, and a few citations must suffice here. Among the earliest are triple-faced heads, the best-known of which was found at Corleck, Co. Cavan. Gaulish Mercury was both triple-faced and triple-phallused. Elsewhere in ancient sculpture, the mysterious hooded figure Genius Cucullatus appears singly among Continental survivals but forms a disciplined trio, Genii Cucullati, in Britain. The Roman poet Lucan (1st cent. ad) proposed that the Gaulish gods Esus, Taranis, and Teutates were mentioned so often together as to form a triad. In early Ireland examples include the three Fothads, the three sons of the Dagda, and Finn Emna or the Three Finns of Emain Macha. There are three female personifications of Ireland, Ériu, Banba, and Fódla, and three gods of craft, Credne, Goibniu, and Luchta. Tlachtga is raped by the three sons of Simon Magus and gives birth to triplets. Ingcél Cáech has three pupils in one terrible black eye. Branwen of the Mabinogi is one of three matriarchs. In Wales there are Three Exalted Prisoners and Three Generous Men of the Isle of Britain. The symbol of the Isle of Man is the three-legged triskelion.
See Joseph Vendryes, ‘L'Unité en trois personnes chez les Celtes’, in Choix d'études linguistiques et celtiques (Paris, 1952), 233–46;Miranda J. Green, Symbol and Image in Celtic Religious Art (London and New York, 1989), 169–205.