The three daily wagers, working for a private company manufacturing plastic covers, were asked by their supervisor to get into the septic tank in the Anuradha industrial estate at Ghodbunder road here for cleaning it yesterday, Kashimira assistant police inspector B G Pawar said.
The victims, who were not trained in doing the hazardous work, got inside the tank and were suffocated to death after inhaling poisonous gases, the police said.
The deceased were identified as - Umashankar Rai (37), Abhiman S Ubale (50) and Suresh Raghunath Jagdandh (60).
Initially, a case of accidental death was registered which was later converted to one under section 304-A (culpable homicide not amounting to murder).
The offence has been registered against the supervisor who had asked the workers to carry out the maintenance work without proper safety norms and work permit, police said.
However, police were yet to arrest the accused in connection with the case.