Following incessant rains for the last few days, the earth was loose in the area and unaware of it they were digging the tunnel.
The thrio were identified as Marimuthu, Arun and Anand of Virudhanagar, aged between 35 and 50 years, police said.
Though their co-workers and public rushed to the spot and managed to clear the soil, only their bodies could be retrieved, police said.
The bodies would be sent to their home villages after conducting postmortem, police said.