The three US troops were hit by shrapnel during fighting on Sunday at the base of the US-led force in Deh Rawood in troubled central Uruzgan province, military spokesman Lieutenent Colonel Bryan Hilferty said in Kabul.
"We returned fire, killing one of the attackers," Hilferty said. The 15 rebel fighters used rocket-propelled grenades and AK-47 rifles to attack the troops, he said.
The direct attack on the US-led forces base from heavy machine gun fire was uncommon as insurgents have mostly used rockets in their attacks against troops, Hilferty added.
He would not speculate on the background of the attackers. Similar attacks have been blamed on remnants of the ousted
Taliban regime, their allies from the al-Qaida network and loyalists of renegade warlord Gulb al-Din Hekmatyar.
The wounded soldiers were air-lifted to the US-led forces' Bagram Air Base 50km north of Kabul and are in a stable condition, Hilferty said.
Like many other bases of the US-led forces in insurgency-hit
central Afghanistan, Deh Rawood, 400km south of the capital, has been frequently attacked by suspected Taliban fighters.
Sunday's clash follows a wave of insurgencies, mainly in southern and eastern Afghanistan, blamed on resurgent Taliban fighters and their al-Qaida allies.
The 12,000-strong US-led force is now in its third year of hunting rebel holdouts, mainly along the 2400km border with Pakistan.