The incident occurred near Davtori village under Bisauli police station area when Amit (22), Mahesh (40) and Sunil (24) were returning home after work on a motorcycle and the bus hit them from behind, leaving the trio dead on the spot, police said.
The bus, a double-decker, is said to have been plying on the route without permit.
The bodies have been sent for postmortem, police said, adding that investigation is underway.