Some 276 young girls were abducted by the Islamist extremists in 2014, shocking the world and sparking a huge campaign with the slogan "Bring Back Our Girls".
More than 20 were released in October in a deal brokered by the International Red Cross and others have escaped or been rescued, but 195 are still missing.
A day before the third anniversary of the kidnapping, President Muhammadu Buhari said the government "is in constant touch through negotiations, through local intelligence to secure the release of the remaining girls and other abducted persons unharmed".
In 2015, the Islamist militants pledged their allegiance to so-called Islamic State.
They have been waging an eight-year military campaign to carve out an Islamic state in northern Nigeria.
Last year, several senior Boko Haram fighters were killed by Nigeria's air force, with the group's leader believed to be among the dead.
Several months later, the terror group was ousted from its last major camp in the northeastern Sambisa forest stronghold.