On one level there’s some intellectual wanking about triangles, then as you point out any shot with thee things will have a triangle. I wonder, though, ignoring any attempts to over-analyze, if three is a nice number of things to have in a shot. The viewer can only focus on so many things after all.

Three is also a sort of especially unspecial number. Zero things is sort of special in the sense that there’s no concentration of focus (shot of a landscape that just establishes the environment). If the focus is on one thing, then that’s really drawing a ton of attention to that thing (camera zooms in on the murder weapon and lingers). Two things can often be focused on the contrast between them (the villain towers over the hero). Three is the lowest number that doesn’t have a ton of baggage.