The idea of Maiden, Mother and Crone don't work well with the Celtic beliefs. There is no true triple goddess in mythology, it is a new age concept created by the poet Robert Graves.
I personally teach my daughter, however, that the Goddess (Mother Earth) gives birth to the Sun and becomes the Great Mother. The Goddess isn't the most important aspect of Yule, the spotlight is on the Sun's death and rebirth. The Goddess is focused on during the sabbats that land in the beginning of the month. The Solstices are always male centered celebration (those celebrated in the middle of the month). After the birth of the Sun, depending on the person, the Goddess goes to sleep or grows old and dies until Spring. So I would say she is the Mother during Yule because a crone doesn't give birth.