This is the fundamental concept in Chinese Metaphysics. It looks awesome but in fact the idea is simple when the metaphysics costume is removed. It simply means Time, Space and Action.
It is not necessary to modify the order placing Man in between Heaven and Earth. In fact, the three are merged together and cannot be separated. Man always does something in a space at a time. How can the three be separated to have the picture of above, below and in between? In Chinese we call this “adding feet to a snake in a painting 畵蛇添足”.