London-based Global Risk Strategies confirmed the fatalities and said its employees had been helping the United Nations arrange elections in eastern Afghanistan.
But UN spokesman in Kabul, Manoel de Almieda E Silva, declined to identify the victims or to give their nationalities and had no details on how they were killed.
However, Taliban commander Mulla Sabir Mumin described the victims as British mercenaries.
Mumin added the three died because the Islamist group targeted all locals and foreigners who "help the Americans to consolidate their occupation of Afghanistan".
NATO concern of a possible spring offensive ahead of parliamentary and presidential elections in September appear to be coming true.
On Tuesday, Taliban fighters killed nine government soldiers and five policemen in south Afghanistan.
Taliban spokesman Haji Abd al-Latif Hakimi said a government militia from Kandahar was ambushed when it entered the remote district of Meya Nishin.
Risky business: US soldiers police
road to Kandahar
Doctors in Zabul province say they received the bullet ridden bodies of five policemen abducted on Monday in the Shah Joy district.
Taliban commander Mulla Rozi Khan confirmed the deaths and took responsibility for them. Four more government soldiers were killed on Sunday in Zabul in a mine blast.
Aljazeera.net's correspondent says the Islamist group is now operating a "classic guerrilla war", with some raiders actually taking over government buildings – if only for a few hours.
On Thursday, anti-government forces took control of a governor's residence and security headquarters in the second largest district of Uruzgan province.
The raid resulted in the death of two government soldiers and the destruction of numerous military vehicles.
Some 700 people have been killed since last August, mostly in raids blamed on the Taliban - which has vowed to push the US-backed government of interim president Hamid Karzai and all foreign troops out of Afghanistan.