Celtic Christianity and Paganism
According the the Celtic beliefs, the most important things in the world came in three, three domains (earth, sea, and sky), three elements (earth, fire, and water), three stages of life (birth, life, death).
This shape is formed by three vesica piscis and a circle to unify them all. There have been different meanings for what the three represent over time. In Christianity it has been said to represent the Holy Trinity of Father, Son, and Holy Spirit. The circle emphasizes unity or eternity.
In more pagan translation, the three vesica piscis represents
the threefold of nature of the Goddess as mother, maiden and crone. The mother is representative of creation, the maiden representative of innocence, and the crone of wisdom.