The incident occurred when one labourer went inside the septic tank to remove the centring woods from there. But he did not come out.
A second labourer then went in and he too did not return, police said.
When a third person also failed to come up from the septic tank, local people informed the police.
The three were later lifted from the tank and taken to hospital where they were declared dead on arrival.
Bodies of the deceased - Baghbool Mondal, Farook Mondal and Abdul Halim Mondal - were sent for postmortem.
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