The attacker riding on a motorcycle detonated his explosives Wednesday at a park in the city of Maimanah, the capital of Faryab province. At least 26 people were wounded in the blast.
The Taliban claimed responsibility for the attack.
Afghan police earlier said that four police officers and six civilians were killed in the attack. Western officials later confirmed that three American soldiers were killed. Faryab's police chief said the troops were filming interviews at the park when the blast took place.
Video footage of the scene showed international troops lying immobile on the ground alongside Afghan civilians and security personnel. Blood was visible on the ground.
So far this year, nearly 100 international troops have been killed in Afghanistan. Violence continues as coalition forces have begun withdrawing from Afghanistan and transferring security responsibility to their Afghan counterparts.
The United States and Afghanistan are also pushing toward completion of a long-term strategic agreement defining the U.S. presence in Afghanistan once all foreign combat troops leave the country by 2014.
The issue of coalition night raids has been an obstacle to the agreement. Afghan President Hamid Karzai has called for an end to such operations, saying they result in civilian casualties and are an invasion of privacy. NATO has maintained the raids are effective in capturing insurgents.
Late Tuesday, U.S. defense officials said the two sides are close to a deal that would allow Afghan forces to take the lead in night raids, with Afghan judges issuing warrants for the operations.
U.S. officials want a deal in place before next month's NATO summit in the U.S. city of Chicago focusing on the future of Afghanistan, including the cost and size of maintaining Afghan security forces.
Afghanistan's minister of commerce and industry, Anwar-ul-Haq Ahadi, told says he is "optimistic" his country will get a positive response from NATO on support for its security needs.
"I think there's been already discussion of $4.1 billion annual assistance to our security forces and I'm quite hopeful that this will get endorsed in Chicago so that for the next 10 years we will have a more reliable source of funding to support our security forces," said Ahadi.
Ahadi said he expects a similar response from a conference in Tokyo in July, and that both gatherings will have an important role in the future of Afghanistan.
Ahadi also said Afghanistan continues to work with the United States on the issue of oil imports from Iran.
The U.S. is preparing to impose sanctions against foreign banks that make oil-related financial transactions with Iran's central bank. The U.S. action, in concert with a planned European embargo of Iranian oil purchases, is aimed at pressuring Iran to abandon its disputed nuclear weapons program. Tehran maintains its nuclear work is for civilian purposes.
Ahadi said Afghanistan is "quite dependent" on Iranian oil and does not yet have an alternative. But he said the Obama administration has been "quite understanding" by allowing exemptions if the supply of oil from countries other than Iran does not allow importers to cut Iranian supplies significantly.