Re: Circle Triangle Square origins
the triangle, circle and square are used quite frequently in all of the eastern traditions (particularly hinduism, buddhism, taoism and shinto), particularly in their esoteric schools. and as you pointed out, they appear to be fairly universal.
a good a place as any to start is in john stevens-sensei's philosophy of aikido , which discusses not only the eastern but western usages, and secrets of aikido where he explains them in the sense of origins and use in aikido. another good book is william gleason-sensei's the spiritual foundations of aikido which discusses them a bit more in terms of esoteric shintoism. also check out saotome-sensei's aikido and the harmony of nature.
hope this helps...