Why use three coppers for divination instead of two coppers?

There is a school of Confucian classics in the Book of Changes, which uses copper coins for divination. Later evolution can also be calculated by numbers. That is to say, the numbers are calculated according to Ganyi, Exchange II, Separation III, Earthquake IV, Xun V, Kan VI, Gen VII and Kun VIII, but there are generally three numbers, one is the external divination of congenital divination, the other is the internal divination of congenital divination, and the third is the dynamic divination.