The idea of San Cai is fundamental in all kinds of Chinese studies. In recent years, some people Christianized the idea and created the term Cosmic Trinity.
The term San Cai first appears in the Ten Wings of the Yi Jing. In Xi Ci Zhuan 繫辭傳, it is said:
There is the Way (Dao) of Heaven, the Way of Man, the Way of Earth.
In Shuo Gua Zhuan 說卦傳, it is said:
To establish the Way of Heaven, it is called Yin and Yang. To establish the Way of Earth, it is called softness and stiffness. To establish the Way of Man, it is called benevolence and righteousness.
The concept of polarity is obvious. Heaven is described as cycles of growing and decaying of darkness and brightness which in turn implies the cycles of time.
Earth is described as the contrasting pair of qualities of materials. This can be extended to cover flexibility and rigidness in a more abstract form.
Man is described as the basic quality of benevolence and the action of righteousness. This also gives rise to the concept of Body and Application, Ti and Yong (體用).
It is clever to associate San Cai with (year, month, day, hour) stars, house gua and ming gua in Feng Shui studies. At the same time it can be looked upon as abuse of the idea of San Cai. The beauty of the concept is totally lost.