The Sanshi (三尸, three insects), or San-chuu, are parasitic yōkai emissaries of Enma-Daiō. The Sanshi appeared only in the "Nezumi-Otoko and Neko-Musume" story. They live inside every person and tattle to Enma if a person is really wicked. Then a red draft paper is send to a yōkai to kill that person.
After hearing about the Sanshi, Nezumi-Otoko decides to construct a "church of longevity" and sell to the mass a fake pill to stop the Sanshi's power. But the Sanshi within Nezumi had tattle to Enma-Daiō and Kitaro was ordered to kill him. Of course, Kitaro did not have the heart to do it, so he ask Neko-Musume to "shake" him up a bit to repent.
Of course in the 1968 anime version of the story, despite Kitaro and Neko-Musume getting Nezumi-Otoko to repent and give back the money to the people he duped, Enma had taken Medama-Oyaji because he (Kitaro) had failed to kill Nezumi-Otoko; also Nezumi's Sanshi had tattled. The only way to save both Medama-Oyaji and Nezumi-Otoko is that he (Nezumi) would fast for two weeks because according to Oyaji, the Sanshi, like normal parasites, live off the nutrients of their host. Take away the nutrients and the Sanshi would die.
According to Kōshin, the Sanshi are green, black and white worms. Actually they look like a man's leg with a cow's head on top, a dog-like beast and a man. The Sanshi ascend to Heaven (not Hell) every 57th day on "Koshin-machi" while their human host sleeps, to tell of the person behavior. A wicked person would try to stay up all night, reciting prayers to stop the Sanshi from leaving the body to tell the Emperor of Heaven.