As I have been thinking about the 9 triads and how they relate to each other, it struck me that the Psychological Triads are closely related to the flow of arrows of all the types to the 3 centers. Read this post for an analysis of the flow of arrows.
The 3-6-9 and 1-8 have a lot in common. They all flow to all 3 centers via arrows. There is more when you look at the Psychological Triads. Image below is from the Enneagram Philosophy tumblr page.
The three triads are the Intensity/Polarity Triad 147, the Superiority/Dominance Triad 258, and the Multiplicity Triad 369.
This means that enneatype 1 is by definition triple intense/polar, enneatype 8 is by definition triple superior/dominant, and of course, Types 3 and 6 and 9 are by definition triple multiplicitous (is that even a word?), or as I like to call them, triple chameleons.