Where was the origination of Ueshiba's usage of the square, circle, and triangle?
Where exactly Ueshiba got his from I cannot say, but I have seen circle, square and triangle appear together with two other symbols (straight line and an L shaped wedge) in a few other ryuha of much older vintage than aikido, so the symbols are not unknown elsewhere. I recall Karl Friday discusses the five in terms of how Kashima Shin-ryu understands them briefly in his book Legacies of the Sword. My own ryu also makes use of the to pass on certain principles. Where Ueshiba adopted them from is another matter though. Perhaps he knew of the original set of five and did away with the other two? Perhaps he got them from somewhere else in just the simple set of three? Maybe he made the whole thing up? (shrugs)