The incident occurred Sunday in Vistahermosa, in Meta province, said the police chief in the region Jorge Nieto.
Nieto said the landmine had been planted by the Revolutionary Armed Forces of Colombia (FARC) on a pathway used by children and sometimes also by police and soldiers.
Two of the minors died instantly, and the other died hours later in a hospital from the wounds suffered, Nieto said.
Vistahermosa residents announced a march Tuesday to protest over the killing of the three children.
In Colombia, wracked by civil war for more than 40 years, some 100,000 anti-personnel mines have been planted by illegal armed groups. In 2003, the landmines claimed 120 lives.
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