Army spokesman Colonel Sani Kukasheka Usman said today that the three were between the ages of 10 and 15 years old.
He said they attacked at a security checkpoint yesterday in Benisheikh, a town in Nigeria's northeast Borno State.
Usman says the army yesterday killed at least 12 Boko Haram fighters in an operation in the same state. He said they raided two camps in the operation.
Boko Haram's 6-year-old Islamic uprising has killed an estimated 20,000 people and driven 2.3 million from their homes, according to Amnesty International and the United Nations.