Sometimes, to provoke more indepth thoughts with something I'll see a number or a shape, or objects and I'll count them quickly and register it with this meaning full list.
0 - Nothing, Off
1 - Unity, Singularity, Consciousness, On
2 - Disassociate, Separation, Duality
3 - Truth
4 - The Bigger Picture
5 - Protection
6 - Life, Carbon
7 - Significance
8 - Infinity "Nothing ( 0 ), with a twist( 8 )", life and sun worshiping
9 - Down side up six (anti life), Fluorine, Evolution, War, Conspiracy
Some of it makes sense.
You know, the interesting thing about 3, and truth, is that in research, particularly social research, if you can support an assertion or concept through triangulation, you are basically considered to have established a "truth". Generally it means using three different kinds of research techniques. (e.g. - surveys, focus groups, and literature review).
So I think your association is pretty appropriate.
And I'm not thinking it is synesthesia, and probably more likely assiciations you've accumulated over the years. Kind of like I tend to associate certain smells or sounds with certain thoughts or feelings, it's because the first time I smelled that smell, or heard that sound, or that song, a certain occurrence, well, occurred.