Unique Interpretations of Trinity Celtic Symbol in Various Cultures

Symbolism of Celtic knot
The Trinity Celtic Symbol, aka the Triquetra, is one of the most common signs from old religious symbolism. Its use spans across multiple religions and many centuries, a large part of which has been lost to time. Yet the symbol seems to get more popular with time.
The oldest known Celts were a group of tribal people that lived in central Europe during the Iron Age. Towards the end of the 6th century BC, the Celts had occupied almost all of Europe, including the Iberian peninsula and the British Isles. Unfortunately for them, the rise of the Germanic tribes and then the Romans resulted in the Celtic tribes being driven out of Central Europe. By the first millennium AD, almost all the Celtic population was concentrated in the British Isles.
So when we talk of Celtic art, culture and symbolism, we are referring to the societies that developed in Scotland and other places such as Cornwall, Isle of Man and Brittany in France. Today, the word 'Celtic' is associated with the Irish and the Scots.
What is The Triquetra?
The symbol can be described as an individual strand that wraps into and onto itself and looks like a three-sided, three-lobed design. The word 'triquetra' itself is derived from the Latin words 'tri-' meaning three and 'quetrus-' meaning corners.

┗ Origins
It's too obscure to verify whether it came from Christian or Pagan beliefs. Most believe it to be Pagan; the Christians later adopted into their religion. We therefore have different meanings to the symbol, based on religion.
The Several Meanings of The Triquetra
Pagan Triquetra
Pagan Celts rarely used the Triquetra individually. It was mostly used along with a bunch of other symbols on carvings and stone tablets. Other similar symbols are the Valknut (three triangles, related to Odin) and the three-horned symbol found on the Snoldelev Stone. Neopagan theories of the Triquetra are usually explained as the three unified feminine powers - mother, maiden and crone. The Celtic knot is usually woven with a circle; this represents the protection of the eternal flow of energy
Christian Triquetra
The Christians use the Triquetra as a symbol of the Blessed Trinity - The Father, The Son and The Holy Ghost. The Christians used this symbol individually and on a much larger scale; they would use it to identify other Christians, along with the Ichthys, the fish symbol.
Modern Use
Triquetra has been very popular over the past two centuries. John Paul Jones, the bass player of the hugely popular rock band, Led Zeppelin, used the Triquetra as his personal symbol, perhaps highlighting his Celtic background. It has also been used in the famous TV series, Charmed, as it represented the power of the three witches acting as one. Opposed to what many people may think, the Triquetra is shown to have no evil undertones whatsoever.
Modern references to this word include the Scottish premier soccer team, Glasgow Celtic and the Boston NBA team, Boston Celtics, where there is a large Irish population.

One of the largest points of attraction towards the Triquetra is perhaps its obscurity. There is no concrete origin or meaning to the symbol, which only makes it more mystical and enchanting. Whether it is used in Pagan, Christian or Wiccan contexts, it seems to carry its own story of magic and religion with it.