Victims were identified as Jugraj Singh, Jagseer Singh and Jujhar Singh, all residents of Ghall Kalan village here, they further said.
The incident occurred on Friday when labourers were called in by a marriage palace owner to clean the septic tank.
Police investigation revealed that one of the labourers first slipped into the septic tank which was about 40 feet deep. When two other labourers climbed down to save him, the rope snapped and they also fell in the tank.
Bodies of labourers were brought out from the septic tank with district administration launching rescue operations.
A case against the owner of the marriage palace identified as Sukhjinder Singh and Manager Sohan Singh have been booked under section 304 (culpable homicide not amounting to murder) of the IPC, police said.
However, no arrest has been made so far.