Hmmm ... I only saw them in a couple of places, and I didn't even find an "articles" section, unless you are referring to the journals. Guess I'm just used to seeing those kinds of symbols connected with spiritual matters, as they do have spiritual connotations.
Thank you for your interest in Daily Zen. These three shapes are
traditional to Zen. You can read many explanations, but for myself I
like to keep it quite simple and will share with you how I view them.
The circle represents empty mind, unpressured mind, open mind; the
square represents the grounded energy of the world, foundation, steady
platform; the triangle represents the focus or concentrated mind of
practice that is necessary to complete training.
Hope this serves you well. It is my "feeling" for the symbols rather
than a literal "official" explanation.