I used a recent visit to London to get shots of implied and actual triangles.
A. Implied triangles

I think this is a bit more subtle as the compression of a zoom lens at 300mm create a group of three people whereas in reality the lady in the middle had no connection with the couple nearest to the camera.

This on the other hand, with what appears to be three generations, is a more connected inverse triangle.
B. Actual Triangles.
C. Still life
In the second part of the exercise you are asked to create two triangular set ups – apex at the top and apex at the bottom.
I decided to use some lenses for the subject and played around with various set ups:-

The low angle creates in my view a more dynamic composition while still creating, albeit less obviously, a triangle.

The in between angle of the above. Again the triangular shape is clear and the spacing of the lenses in the frame is better I think although arguably the picture is not as dynamic.
D. People
These two shots are from a studio shoot I did for my girlfriend’s birthday with her five sons. During the session there were a number of arrangements but these two represent the most “triangular.”
I think I tend to spot these compositional shapes, be they rectangular circular or triangular, on an almost subconscious level at the point of shooting and then more proactively at the selection/weeding out process.