Be careful with the concept of “the” Triple Goddesses. As I understand it, there were goddesses who were threefold, but not three separate goddesses in one, if you follow me. So Brigit is “a” triple goddess in that she has three aspects – sometimes described as sisters. The Morrigan also has three aspects, although they vary, and then there are the Gaulish Matronae, perhaps connected to the British/Welsh Modron, who may have been three aspects of the same divinity – or sisters. In Athurian legends we even find the three Gwenhwyfars (Gueniveres), albeit rather confusingly. Ériu, Banba and Fódla are three sisters, but they are distinct. We also find triplicity with male divinities, notably Lugh. The traditional materials are by no means consistent and the “obsession” of ancient Celtic spirituality with triplicity is well noted. I don’t think that there was one Tripe Goddess in ancient Celtic belief systems, but rather divinities that manifested themselves in threes. As for the maiden-mother-crone idea, it seems we have Robert Graves to thank for this, as we do for the idea of a Holly King if I’m not mistaken, and it is certainly not backed up by ancient sources.