The anti-talk insurgent faction opened fire on a truck carrying construction materials for a bridge in Mornoi Tea Estate area along the Assam-Bengal border around 12.45 pm, police said.
While the truck driver and two labourers were gunned down on the spot, two other workers were seriously injured, the police said.
The driver was identified as Rajib Mushahary and the labourers as Asatun Bibi and Ashabul Sheikh.
The injured, identified as Ayuf Rahman, was admitted to hospital.
Kokrajhar constituency will to the polls on April 24.