The three were killed by suspected rebels traveling by motorcycle in Tibu, near the Venezuelan border, police said in a statement.
The suspects had earlier attacked a police station in the same area, but nobody was wounded, the statement added.
The Norte de Santander department, where Tibu is located, is one of Colombia's biggest areas for coca growing, and several drug-trafficking factions operate in the area.
The ELN began peace talks with President Juan Manuel Santos's government a year ago in the Ecuadoran capital Quito, but he suspended negotiations at the end of January and ordered a stepped-up military response after bomb attacks blamed on the guerrillas.
Relations between Colombia and Venezuela, which share a 2,200 kilometer border, are tense as Colombia deals with the arrival of thousands of Venezuelans fleeing the country's economic and humanitarian crisis.
According to Colombian immigration authorities, around 550,000 Venezuelans are in the country -- a figure it said could rise to one million by mid-2018.