The incident occurred when the house owner went into the tank yesterday to clean it and two others followed suit when the former failed to resurface, the officer said.
A fourth person who went to look for the other three, was rescued in an unconscious state, the officer said.
All the four persons were brought out of the tank by villagers and taken to hospital where three were declared dead.
The deceased were identified as Hari Behera (48) the house owner, and his relatives Mina Behera and Chandra Sekhar Guru, the officer said.
Police have registered a case of unnatural death investigation was on, while post-mortem on the bodies had been conducted, the SDPO said.