Two were working for the International Organization of Migration, and one was a contracted medical doctor working for UNICEF.
The Nigerian government announced today that the search for the 110 schoolgirls who were abducted from the Government Girls Science and Technical College (GGSTC), Dapchi, Yobe State, has been extended to the neighbouring countries of Chad, Niger and Cameroon.
Samantha added that the United Nations was concerned about other civilians who might have been injured or killed in the attack.
The whereabouts of two midwives are also unknown, according to an International Committee of the Red Cross spokeswoman, who said no Red Cross staff were injured or missing.
Boko Haram aims to create an Islamic state in northeast Nigeria.
More than 20,000 people have been killed, and over two million have been forced to flee their homes since 2009 when the group waged an armed campaign in northeastern Nigeria.
Rann plays host to a camp for internally displaced persons.
IOM official Mohammed Abdiker said staff were "outraged and saddened" by the death of their colleagues. "Humanitarian workers should never be a target", she added.
The kidnapping and continued attacks in the region have raised questions about the extent of the Nigerian government's claims to have virtually defeated Boko Haram.
So far, the Nigerian air troops had already flown more than 200 hours while conducting the search.
In January 2017, a botched Nigerian airstrike meant to hit jihadist fighters killed at least 112 people as aid workers distributed food.