Their veneration began dates goes back to the beginning of religious Taoism. Their help was enlisted during healing ceremonies by both Zhang Jue and Zhan Dao-ling, the respective leaders of the Way of Supreme Peace and Five-Pecks-of-Rice Taoism. During these healing sessions, people would write their sins -- believed to be the cause of their illness -- on three pieces of paper, one for each ruler. The paper intended for Tian-guan was either burned or deposited on the peak of a mountain; the paper addressed to Di-guan was buried in the ground; and the one for Shui-guan was thrown in the water.
The cult of San-guan was reformed under Kou Qian-zhi and they became functionaries of the gods, supervising rites and rewarding believers. Each city had a hall or temple dedicated to them. They are not often portrayed together, but when they are they sit next to each other and are dressed as mandarins.