Acting on a tip-off, the police, army and CRPF personnel had launched a joint operation in Dolamara area, under Santipur police station in Bokajan sub-division of the district, and came upon a group of seven heavily armed militants.
A fierce encounter ensued in which three militants were killed but the remaining four managed to escape, police said.
The security forces recovered three AK-56 rifles and huge quantities of ammunition from the encounter site.
A massive combing operation has been launched in the area to nab the militants who managed to escape, the sources added.