Triquetra

Celtic art
The triquetra is often found in insular art, most notably metal work and in illuminated manuscripts like the Book of Kells. It is also found in similar artwork on Celtic crosses and slabs from the early Christian period. The fact that the triquetra rarely stood alone in medieval Celtic art has cast reasonable doubt on its use as a primary symbol of belief. In manuscripts it is used primarily as a space filler or ornament in much more complex compositions, and in knotwork panels it is a design motif integrated with other design elements. This widely recognised knot has been used as a singular symbol for the past two centuries by Celtic Christians, pagans and agnostics as a sign of special things and people that are threefold.

Japanese use
The triquetra has been a known symbol in Japan called Musubi Mitsugashiwa. Being one of the forms of the Aryan Iakšaku dynasty signs, it reached Japan with the dynasty's Kāśyapīya spreading technology and Buddhism via Kingdom of Khotan, after which the Japanese sword katana is called, China and Korea.

Germanic paganism
The triquetra has been found on runestones in Northern Europe and on early Germanic coins. It presumably had pagan religious meaning and it bears a resemblance to the valknut, a symbol associated with Odin.

Christian use
The symbol has been used in Christian tradition as a sign of the Trinity (Father, Son and Holy Spirit), especially since the Celtic revival of the 19th century. When modern designers began to display the triquetra as a stand-alone design, it recalled the three-leafed shamrock which was similarly offered as a Trinity symbol by Saint Patrick have also suggested that the triquetra has a similarity to the Christian Ιχθυς symbol. The triquetra has been used extensively on Christian sculpture, vestments, book arts and stained glass. It has been used on the title page and binding of some editions of the New King James 
A very common representation of the symbol is with a circle that goes through the three interconnected loops of the triquetra. The circle emphasises the unity of the whole combination of three forces. It is also said to symbolise God's love around the Holy Trinity.
Source: https://en.wikipedia.org/wiki/Triquetra