The security personnel, who were patrolling Nongpok Keithelmanbi area, were fired upon by suspected Kuki National Front (Nonlun) faction militants in the early hours today.
The security personnel returned the fire during which the three militants were killed, the sources said.
Additional security forces from the district headquarters in Thoubal have been rushed to the spot.
Official reports from the district headquarters said that the exchange of fire was still continuing.