I think it's a very deeply rooted and ancient phenomenon, to think in threes. Georges Dumézil theorised about what he believed to be the 'tripartite' organisation of Indo-European cultures. Obviously in so-called 'Celtic' culture it's very prominent - not just in the Welsh triads but equivalent early Irish texts, the symbol of the triskelion and the notion of the 'triple goddess' a deity with three contrasting yet complementary aspects. Also the Greek Fates, Norse Norns, etc. Countless examples of this phenomenon across time and cultures.