article source: http://www.byzant.com/Mystical/Symbols/Triquetra.aspx
A triquetra is a figure formed from three circular arcs of equal radius, producing three overlapping vesicae piscis (pointed ovals). It is usually enclosed or intersected by a circle, as in the example above, and is often seen in Gothic and Celtic art.
The triquetra is primarily a symbol of trinities. It is most commonly used in a Pagan or Christian sense to represent respectively the triple aspects of the Goddess (Maiden, Mother and Crone) or the trinity of Father, Son and Holy Spirit. The unbroken circle symbolizes the essential unity of whatever triad is being signified by the three outer points and the central, curved triangle. Many three-fold systems can be represented by the triquetra, such as life, death and rebirth; the Taoist concepts (yin, Tao and yang); the tantric energy channels (ida, sushumna and pingala); the Hindu trinity (Shiva, Brahma and Vishnu); the alchemical concepts (Sulfur, Mercury and Salt); the phases of the moon (waning, full and waxing); and the Hebrew "Mother" letters (Mem, Aleph and Shin).
Three shown on a circle represents Creation based on Triplicity. Nothing exists but is a 'Three', composed on the basic elements of Life: Spirit, Soul and Mind, and applies to everything though some inert in aspects of primitive forms.