The Sanchū (三虫 Sanchū, lit. Three Insects) are parasitic yōkai emissaries of Enma-Daiō. The Sanchū 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 Sanchū, Nezumi-Otoko decides to construct a "church of longevity" and sell to the mass a fake pill to stop the Sanchū's power. But the Sanchū within Nezumi had tattle to Enma-Daiō and Kitarō was ordered to kill him. Of course, Kitarō did not have the heart to do it, so he asked 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 Kitaro had failed to kill Nezumi-Otoko; also Nezumi's Sanchū had tattled. The only way to save both Medama-Oyaji and Nezumi-Otoko is that he would fast for two weeks because according to Oyaji, the Sanchū, like normal parasites, live off the nutrients of their host. Take away the nutrients and the Sanchū would die.
The Sanchū are based on the Sanshi (三尸 Sanshi, lit. Three Corpses) from Chinese Taoism. 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 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.