The incident happened at Mangachak village where the three men were tilling their fields in the heavy downpour that lashed the area, they said.
The lightning fell on them and they died on the spot and a boy standing nearby sustained burn injuries, they added.
Those deceased have been identified as Deonandan Yadav, Shiv Nandan Yadav, Yugal Yadav, all were cousins belonging to the same family, police said, adding that their bodies have been sent for post mortem.