The insurgents attacked the soldiers from the area of a school on the outskirts of Kandahar city, a NATO statement said. Six other soldiers received non-life threatening injuries, it said.
In separate attacks Thursday, one Canadian soldier from the NATO force was killed and four wounded in roadside bombings, also near Kandahar.
The statement did not identify the nationality of the latest fatalities.
Earlier Thursday, a suicide bomber in a car blew himself up in a crowded town market in southern Afghanistan on Thursday, killing 21 civilians near a NATO convoy, officials said.
Thirteen people were also injured in the blast, which left a scene of devastation in the heart of the town of Panjwayi in Kandahar province, said provincial government spokesman Dawood Ahmadi.
Some of the victims were children, said Interior Ministry spokesman Yousef Stanezai.
Nasim Jan, a local police official, said it was a suicide attack. He blamed the Taliban.
The attack, one of the deadliest bombings in Afghanistan since the ouster of the Taliban in late 2001 by U.S.-led forces, came just days after NATO took charge of security in the volatile south from the U.S.-led coalition.
The bomber in Panjwayi blew up the car 200 yards from where three Canadian vehicles in a logistics convoy were parked, a spokesman for Canadian troops in Kandahar, Maj. Quentin Innis, said.
The armored vehicles were not damaged but were close enough to feel the force of the blast, he said.
Innis said it was unclear whether the attacker was targeting the civilians in the market or the convoy.
At least 14 shops were burned by the blast, which left a crater about 5 feet across and 1.5 feet deep. The wreckage of the destroyed car was flung nearly 100 yards toward the district chief's office. Body parts and debris littered the road.
Witness Syed Mohammed Rasool, who owns a motorcycle repair shop damaged in the bombing, said he had been talking to a customer who walked out of his store just before the bomb went off.
"A car stopped and exploded. I came out. The guy who had been standing next to me was dead. There were people who had lost their legs and hands. There were people on the ground crying," Rasool said.
The blast happened at around 2:30 p.m. when the market was busy, said Ahmadi. He said authorities hadn't yet established the identity of the suicide bomber.
Last month, Canadian troops were involved in a major offensive in Panjwayi district, regarded as a Taliban stronghold and an area where many farmers cultivate opium.
The hard-line Islamic militia has stepped up attacks this year, increasingly using suicide bombings, in a shift of tactics reminiscent of the bloody insurgency in Iraq.
On Jan. 16, a suicide bomber on a motorbike killed 20 people in Spinboldak, a border town in Kandahar province on the border with Pakistan. The bomber had driven into a crowd of about 100 people watching a wrestling match.
On June 1, 2005, a suspected al Qaeda fighter detonated explosives strapped to his body in a mosque in Kandahar city, killing 20 worshippers, including a senior Muslim cleric who had urged people not to support the Taliban rebels.