The concept of the tripple goddess today is a very prevelant and popular as well as imporant beleif most particuarly in the Wiccan and Neo-Pagan beleif system whom often worship and hold the figures of the Madian, Mother and Crone.
Triads of deities are quite old and common, and date all the way back to Hindisim one of the worlds oldest religons, whom hold what is refered to today as the Hindu Tritnity, which consists of Brhama (The Creator) Vishnu (The Preserver) and Shiva (The Destroyer)
There are also many examples of Goddesss comming in a traid in several traditions. The Greeks had the Moirae (The Fates) the Charities, also known as the Graces and the Erinnyes (The Furies)
In Norse Mytholgy they had what were known as the Norns whom serve very much the same role as the Fates in Greek Myth
The Irish Celts had Brighid, whom is said to have also had two sisters by the name of Brighid, or the Morrigan whom has at least three or four other names.
In most cases the tripple Goddess are seen as three sperate dieties whom preform different and yet related funtions.
One thing that it is imporant to keep in mind and remeber, is the fact that the popurlized version of the tripple goddess today, as mentioned before the Madian, Mother and Crone, are often seen in relation to and associated with Celtic Traddition, and though the idea of a goddess traid does apepar in Celtic traddition, they never acutally had a Maiden, Mother, and Crone goddess, this idea is new and unique to Wicca.
The Maiden
represents enchantment, inception, expansion, the promise of new beginnings, birth, youth and youthful enthusiasm.
The Mother
represents ripeness, fertility, sexuality, nurturance, fulfillment, stability, power and life.
The Crone
represents wisdom, repose, death, and endings.
Though one might be able to find in varrious older cutlures goddess that could easily represent and fit into the roles of the above. For example for the Greeks, Artimis could the the Maiden, Hecate the Mother, and Gaia as the Crone and so forth, there is no record of any traditional beleifs making any such connections as these.