Wiccan theology as it is normally set out holds that there is one Goddess and that the three aspects are ways of approaching her rather than three beings. That is why the Mother can be addressed at a full moon in one month and the Crone at a waning moon in the next without any suggestion that a different deity is being invoked. The scheme's coherence is the reason it spread so widely, and its history is documented rather than mythical.
Robert Graves set out a goddess in three persons tied to the lunar phases in The White Goddess in 1948, drawing on a wide and highly selective reading of unrelated mythologies. Gerald Gardner and Doreen Valiente put the Goddess at the centre of Wiccan liturgy through the 1950s, and Starhawk's The Spiral Dance of 1979 carried the Maiden, Mother and Crone framing to a much larger audience and shaped the feminist leaning strand of the tradition. Ronald Hutton's The Triumph of the Moon, published in 1999, is the standard history of that sequence, and it treats the scheme as a twentieth century construction rather than a survival from an older religion.
Ancient religions did have goddess triads, and some goddesses were addressed with three faces or three names. What they did not have, on the surviving evidence, is this particular arrangement of Maiden, Mother and Crone matched to the waxing, full and waning moon. Modern Wiccans who know that history generally keep using the aspects anyway, on the grounds that a ritual category does not need to be ancient to work.