"Three coalition service members were killed in an attack on their convoy in Kapisa province this morning when their convoy struck an improvised explosive device and came under small-arms fire," the military said.
The force's media office told AFP separately that the three were from the United States.
There was no immediate claim of responsibility but militants from the radical Hezb-e-Islami faction led by former prime minister Gulbuddin Hekmatyar are active in the area.
Wednesday's killings would take to 126 the number of international soldiers to lose their lives in Afghanistan this year, most of them in attacks, according to a toll by icasualties.org which tracks the wars here and in Iraq.
Nine US soldiers and a Briton have been killed in attacks in the first four days of this month, according to military statements.
The Taliban, which leads the insurgency, warned in April that attacks would increase across Afghanistan, with Afghan and international troops the target as well as officials.
Fighting has escalated since then with at least a dozen people, including insurgents, reported killed almost daily in past weeks. Twenty-seven were reported killed on Wednesday.
There have also been an increasing number of attacks outside the main battle zones of the south and east of the country.
The violence has raised concerns about stability ahead of key presidential and provincial elections on August 20 for which Western countries have pledged thousands more troops.
There are close to 40,000 US soldiers in Afghanistan, forming the bulk of roughly 70,000 foreign troops deployed to the war-torn nation.
The number of US troops is set to rise to 68,000 by the end of the year, still around half the number of foreign soldiers deployed to Iraq.
US President Barack Obama decided to deploy 17,000 combat troops, most of them to the south, in the hope of turning the tide in the gruelling fight against a growing insurgency that is costing the US millions of dollars a day.
The United States led the invasion that removed the Islamist Taliban from government in 2001, weeks after the September 11 attacks blamed on their allies in the Al-Qaeda network which had bases and training camps in Afghanistan.
The extremists were able to regroup, some in sanctuaries across the border in Pakistan, to launch an insurgency that last year reached its deadliest with an estimated 2,200 civilians and 5,000 militants killed.
