By Ben Farmer in Kabul
Akbar Agha was sentenced to 16 years after abducting the trio from the Afghan capital in 2004 and threatening to execute them unless Taliban prisoners were released.
His family said he was quietly freed after a presidential pardon last summer.
Hamid Karzai's palace and the attorney general said they had no information on any release, but an international official told The Daily Telegraph he had apparently been let out because of his poor health.
Disclosure of the pardon came as the White House threatened to withdraw an invitation to Hamid Karzai amid ongoing anger at his accusations foreigners were responsible for last year's widespread electoral fraud.
RAWA: Ghairat Baheer, an infamous terrorist of Islamic Party of Gulbuddin was freed by Afghan government in May 2008 in a move for reconciliation with all enemies of Afghan people.
His allegations have provoked sharp anger in Washington, which is spending billions a year propping up Mr Karzai and last year lost 316 troops fighting the Taliban.
David Miliband, British foreign secretary, called the claims "malign".
Mr Karzai has refused to withdraw the attack and later heightened frustration in the White House when he said the Taliban would become a legitimate resistance movement, which he would join, if foreign meddling continued.
Robert Gibbs, White House press spokesman, said: "We certainly would evaluate whatever continued or further remarks President Karzai makes, as to whether that's constructive to have such a meeting, sure." "Our position on this is that when the Afghan leaders take steps to improve governance and root out corruption, then the president will say kind words," he continued.
"When leaders need to hear stern language from this administration about the consequences of not acting, we'll do that as well."
Waheed Omer, spokesman for Mr Karzai, told a weekly press conference on Wednesday the two allies had often disagreed, but would continue to co-operate.
He said: "[Karzai's remarks] did not have any effect on the strategic relations with the United States and the international community," He went on: "We should not always expect there's agreement. Sometimes there's disagreement." He would not comment on the pardon of Akbar Agha, from Kandahar, who commanded a short-lived Taliban splinter group called Jaish-e-Muslimeen, or Army of Muslims, until his arrest after the kidnap.
The faction claimed responsibility for the kidnap of Annetta Flanigan, a 43-year-old British-Irish citizen; Shqipe Hebibi, 36, a Kosovan woman and Angelito Nayan, 34, a Filipino diplomat, who were working on Afghanistan's first presidential poll.
They were released unharmed more than three weeks later.
The group had threatened to execute their captives unless American forces left Afghanistan and 26 Taliban and al Qaeda prisoners were freed Akbar Agha, believed to now be around 53, was later arrested during a raid on an apartment block in Karachi, in Pakistan.
Mr Karzai has previously faced international criticism for using his presidential powers to pardon drug traffickers and Taliban with powerful political connections.
The new hard line deputy head of the Taliban was mysteriously released from Afghan custody in 2008 after being handed over from Guantánamo Bay prison the previous year.