All three died Saturday, making a total of five NATO deaths that day.
The two identified as American died in an insurgent attack, while the third was killed by a bomb blast, NATO said in statements. The nationality of the third service member was not immediately available, nor were any further details provided.
The Danish government previously announced two of its soldiers were killed when their vehicle struck a bomb in southern Helmand province.
This has been a particularly bloody summer for NATO forces in Afghanistan amid a troop surge and a vigorous Taliban counteroffensive. June was the deadliest month for NATO forces of the nearly 9-year war with 103 killed. For the U.S. contingent alone, July brought the highest-ever death toll of 66 killed.
Including the latest deaths, 11 NATO service members have been killed so far in August.