These pictures show the three and four legged cauldrons displayed in all ancient Chinese temples. The number four stands for the material world so the four-legged cauldron is always placed just inside of the outer door at the junction between the outer material world and the inner sanctum of the temple. The number three stands for the balance between Yin and Yang and represents Heaven. Therefore, the three-legged cauldron is always placed directly in front of the most inner chamber of the temple where Heaven and the spirits emanating from it are to be contacted.
The relevance of these ideas to acupuncture are explained in detail, especially in regard to ancient Chinese alchemy, in my "Pointnames" book. In fact, the main theme of this particular book relies on the the alchemical transformation symbolized by these ancient cauldrons to construct an original theory on how the acupuncture merdians and the acupuncture points on them transform the Qi according to the three ancient levels of Qi transformation from Jing Qi (the Reserve Qi), to Qi (primarily the Qi circulating through the acupuncture meridians), to the Shen/Spirit (conscious awareness) residing in the Heart.
For more information on this book