NATO spokesman Col. Thomas Collins said the blast hit the American convoy in the Zhari district of Kandahar province, the spiritual heartland of the Taliban and one of the most volatile regions in Afghanistan.
Collins originally said four US troops were killed in the blast, but Capt. Luca Carniel later said NATO had revised the death toll to three US service members.
The attack follows a truck bombing a day earlier on a NATO outpost in Helmand province that killed three Georgian soldiers. So far this year, 58 international service troops have been killed in Afghanistan, according to an Associated Press count. Of that tally, 44 are U.S. service members.
Earlier Tuesday, a bomb hidden in a parked motorcycle ripped through a packed market in the village of Safar in Helmand, according to Omer Zawak, the spokesman for the provincial governor. Three people were killed and seven were wounded in the blast, he said.
Four children were among the wounded, two critically, police spokesman Shah Mahmood Hashna said.
Meanwhile, the Taliban released the last four of eight Turks taken hostage last month.
In an email statement, Taliban spokesman Zabiullah Mujahed said the group freed the four Tuesday afternoon as a "goodwill" gesture to fellow Muslims. Turkey's president Recep Tayyip Erdogan confirmed their release.
The eight Turks, along with an Afghan translator and two pilots, were captured after bad weather forced their helicopter to make an emergency landing in eastern Afghanistan on April 21.
The Taliban released the first four Turks of the group on Sunday.
Mujahed did not make any reference to the fate of the Afghan translator and two pilots - one from Russia and one from Kyrgyzstan.
Gannon reported from Kabul. Associated Press writers Amir Shah and Rahim Faiez in Kabul, and Mirwais Khan in Kandahar, Afghanistan, and Misha Dzhindzhikashvili from Tbilisi, Georgia contributed to this report.