The Triple Goddess is a concept in which the Goddess is split into three aspects - Maid, Mother, and Crone. These aspects correspond to the three phases of the moon - waxing, full, and waning. Of course, the moon has a fourth phase, the New Moon, and for this some speak of a Dark or Unseen Goddess. Those who do not use the fourth phase roll the Dark Goddess in with the Crone.
The goddess of Gardner was certainly a moon goddess, and presumably Gardnerians view her as having a triple aspect. This does not mean, however, that every goddess is a triple faced moon goddess, nor that every goddess can be conveniently fitted into one these three aspects. The Triple Goddess can be a template, helping us better address specific aspects or issues while reminding us that despite all of our differences, we are part of a greater whole. Like all concepts, however, we should not lock our beliefs concerning them into immutable rules. Goddesses may be divided among the three aspects, but few goddesses exist solely in one aspect.
Attributes commonly ascribed to the Triple Goddess' aspects:
Maid - Childhood, adolescence, beginnings, purity, virginity, independence, courage Mother - Motherhood, protection, fertility, growth, sexuality Crone - Old age, wisdom, change, transformation, death, rebirth, banishing
The Celts believed in a single Goddess split into three aspects, which the Christians stole as their trinity."I'm so glad I wasn't drinking anything when I first read this. I've debated giving it multiple bunnies.
The Celts viewed many of their gods and goddesses in triple form. However, they were not divided into maid, mother, and crone. The Morrigan, for instance, was comprised of Badb, Macha, and Nemain, all goddesses of war and death. There is some association with sexuality and childbirth as well. These aspects, however, cannot easily be attributed to specific members of the triad - if forced, I'd say Macha might be Mother, with Babd and Nemain being Crone. There is however, no Maid aspect.
And, for the record, we have records of over 400 Celtic deities. The suggestion that they were monotheistic is ignorant at best, or else propagandistic. You cannot just reverse apply Wiccan concepts to historical mythologies because it sounds cool.
The Triple Goddess as represented in Wicca is a Wiccan concept. Pagan cultures did not divide their goddesses up as Maid, Mother, and Crone, nor did they generally envision specific goddesses as having maid, mother and crone aspects. Hekate is perhaps most spoken of in these terms by Wiccans, and it makes the Hellenists spit venom. (To be fair, Ronald Hutton has indicated a short span of time where a group of people did view Hekate in this manner, but that example is very much the exception, not the rule, in regards to historical people's views of Hekate.)
The Triple Goddess...was a synthesis by Robert Graves in The Witches Goddess of the traditional nine Goddesses of Greek and Roman origin. Generally triple goddesses were found as three maids, three mothers or three crones. But never as maid, mother and crone.
There are many female trios in various mythologies. That does not mean they are a Triple Goddess. The one group I can think of that does fit the pattern is the Three Fates of Greek mythology - one who spins the thread of life, one who weaves thread, and one who clips the thread at the end of life. They are not one goddess, however, but three closely connected goddesses. One example, or even a handful of examples, however, does not prove that they were all modeled on some older, greater goddess.