I have this symbol sketch:
Does it have an existing meaning?
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Both symbols have seen a lot of use, as especially the Triquetra Wikipedia article confirms. Among others, it was a Celtic symbol adapted by Christianity.
This looks (to my eyes at least) rather like a very basic Celtic Knot aka a Triquesta or Trinity Knot. There are lots of references to these things to be found on the internet if you search for the above words and phrases.
This is a very old symbol so the original meaning can be a bit lost and different people attribute different meanings to them. A commonly accepted meaning is that the three points / loops indicate mind, body and soul. Also, the fact that Celtic Knots have do ends is often taken to represent the never-ending cycle of life.
"Trefoil" is a name for a sort of universal three-lobed shape that is based on a type of plant, like a clover. Google's definition and images are here:
Because it is such a basic, simple shape, you see it expressed in everything from architectural decoration (think gothic church windows) to the symbol for radiation danger.