U.S. Col. Brian Tribus, a North Atlantic Treaty Organisation spokesman, confirmed the three contractors had been killed, without elaborating.
The bomb detonated near the city’s private Shinozada hospital.
No group including the Taliban militants has so far claimed responsibility behind the incident.
The bombing killed at least nine Afghan civilians and one foreigner and wounded 60, said Wahidullah Mayar, a Health Ministry spokesman.
There were no immediate reports of casualties but an AFP photographer saw the mangled wreckage of a burning vehicle at the scene of the blast, which struck outside a civilian hospital. It wasn’t clear how many armored cars were in the convoy, though it is at least two, often three because of heightened security concerns in the capital.
The scene of the attack is also located close to the US embassy and global Security Assistance Force (ISAF) headquarters, which triggered an alarm immediately after the explosion.
US and North Atlantic Treaty Organisation troops ended their combat mission in Afghanistan last December, though some 13,000 troops have remained to train Afghan forces.
The Taliban are stepping up their summer offensive, launched in late April, amid a bitter leadership dispute following the announcement of the death of longtime leader Mullah Omar.
On August 7, a Taliban attack on a North Atlantic Treaty Organisation military base near Kabul’s worldwide airport killed an American soldier and eight Afghan contractors. The auto bomb caused extensive damage to nearby buildings and cars.