Human rights activists have criticised the United States for the indefinite detention of the roughly 505 detainees being held at the prison for foreign terrorism suspects at the US naval base at Guantanamo Bay, Cuba.
The Pentagon on Friday said the three would be permitted to observe operations at Guantanamo "and ask questions of the command, staff and US officials who would accompany them".
But Lieutenant Colonel Mark Ballesteros, a Pentagon spokesman on detainee issues, said they would not be allowed to speak to detainees because that was the role of the International Committee of the Red Cross (ICRC).
The Pentagon said the invitations were extended to Austria's Manfred Nowak, special investigator for the United Nations on torture, Pakistan's Asma Jahangir, who focuses on religious freedom, and Algeria's Leila Zerrougui, who looks into arbitrary detention.
"This goes to our desire to show that we have nothing to hide," Ballesteros said.
UN investigators previously had sought to visit the facility. UN spokesman Ari Gaitanis said the world body had not immediately received word from its Geneva offices as to whether the three investigators would accept the invitation.
"Although department policy does not provide for such visits to military detention facilities, the department has determined on an exceptional basis to extend this invitation," the Pentagon said in a statement. "The department strives for transparency in our operation to the extent possible in light of security and operational requirements and the need to ensure the safety of our forces."
Criticism by human rights groups has escalated in recent weeks with the US military's disclosure that it was force-feeding Guantanamo detainees staging an ongoing hunger strike about their conditions and lack of legal rights.
The Pentagon has defended its treatment of prisoners and denied that torture has occurred at the Guantanamo facility, which opened in January 2002, just months after the 11 September 2001 attacks on the United States. Most of the detainees were seized in Afghanistan.
Men who have been released from Guantanamo have stated they were tortured there. The ICRC last year accused the US military of using tactics "tantamount to torture" on Guantanamo prisoners. An FBI agent wrote in a memo that became public last year that Pentagon interrogators used "torture techniques" at Guantanamo.