Which three walls do you mean?

Sanyuan refers to Ziweiyuan, Taiweiyuan and Tianshiyuan. Purple element includes the sky area near the north celestial pole, which is roughly equivalent to the area of arch star; Ziweiyuan is the middle wall of three yuan, living in the center of the northern sky, so it is also called Zhonggong, or Ziweiyong. Taiwei Courtyard contains some constellations such as Maiden, Houfa and Lion. Taiwei Courtyard is the upper wall of Sanyuan, located in the lower northeast of Ziwei Courtyard. Shi Tian City Wall contains some constellations such as Snake Man, Immortal, Giant Snake and Eagle. Shi Tian Wall is a three-dimensional lower wall, which is located in the southeast direction under Wei Zi Wall. Sanyuan and the twenty-eight lodges on the ecliptic are collectively called Sanyuan and Twenty-eight lodges.