Also keeping in mind the various Koryu that Takeda Sokaku studied that might have brought influence on how he "mentally mapped" his experience. Jikishinkage Ryu comes to mind as one of those that, if memory serves, has some Ten Chi Jin nomenclature as well as other attributes that might have been relevant and/or influential on Takeda Sokaku and therefore Daito Ryu.
It is my understanding that Ten Chi Jin was commonly used nomenclature used broadly (reminds me of the Sho Chiku Bai conversations that went way over my head quickly, this one could easily do so as well). However, it is also my understanding that the generally used Ten Chi Jin terms could be used by various groups (not necessarily just martial) to refer to specifically nuanced concepts or phenomena uniquely significant unto them.
The whole thing kind of brings the various Greek secret schools to mind. While they discussed and explored universal phenomena (geometry, astronomy, etc.) their views, nomenclature, etc. were specific unto themselves and "held close to the chest" and their explorations were often at once empirical, philosophic, and/or religious. Okay . . . I'm speaking way beyond my "pay grade" now!
Once again, I was, and am, simply suggesting that the subject is deserving of further exploration. Just because one has learned a "Pythagorean Theorem" (and for those feeling left out, just because one speaks and reads ancient Greek) does not mean one understands the totality of the Pythagorean mystery school, any more than dressing up in robes and sandals would.
And the mystery is pretty deeply ingrained, with many reserved interpretations in specific applications.
Setting aside the ninja scrolls, here are some other places one finds the Ten Chi Jin or Ten Jin Chi or Sansai "principle:"
Basic Japanese Literature
And if that isn't enough meat to chew on, consider the folk etymology of the character for emperor:
In that interpetation, the vertical stroke is said to represent the person of the emperor, who unites the three horizontal strokes, which represent (from top to bottom) heaven, humanity, and earth.