Trefoil
The unity of the Persons in the one Godhead is sometimes represented by intersected triangles, or by the trefoil placed under a triangle.
The truth of the Trinity is also suggested by any threefold arrangement in the various forms of the ornamentation.
The figure known as the triquetra, made by the interlacing of three portions of circles, is also symbolical of the Holy Trinity. This is a very ancient emblem, and is found with frequency upon the stone crosses erected in the early days of Christianity in Great Britain. It is sometimes used in ornamentation of the dress of our Lord or of the Evangelists.
Intersected triangles. Trefoil placed under a triangle. Trequetra.
From the thirteenth century we have the symbol of the equal and interlacing Circles. "The three equal circles symbolize the equality of the three Persons in the Trinity, the binding together in one figure the essential unity, while the circular form signifies a never-beginning, never-ending eternity." The word trinitas, used in this symbol, may itself be divided into three syllables. One of these syllables is placed in each circle; but they have no perfect meaning, and will not form any word, unless united. In the space left vacant by the intersection of the circles the word unitas is placed.