The fourth wounded USA service member was treated in country, the Pentagon said.
The U.S. troops were there to provide advice, assistance and aerial surveillance, the statement explained.
One US service member reportedly received sufficient medical care at the scene and three others were transported out of the area to receive treatment.
One coalition service member was wounded.
For several years, USA special operations troops have been on the front lines in Somalia, advising local troops in a long running campaign against the al-Qaida aligned al-Shabab group.
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A US Department of Defence (DoD) representative said one US solider was killed. The US had pulled out of the country after 1993, when two helicopters were shot down in Mogadishu and bodies of Americans were dragged through the streets.
"This attack comes just as the USA military and the White House are reconsidering whether there should be as many special forces, not just in Somalia but in sub-Saharan Africa, because Washington is looking at trying to perhaps shore up its defences in other parts of the world where they see some sort of security threat from Russian Federation and China", she added.
President Donald Trump shared his condolences in a Twitter message Friday evening. Al-Shabab has been blamed for the October truck bombing in Mogadishu that killed more than 500 people. Some of al-Shabaab's top leaders have been killed in US drone strikes, but its operations have continued in Somalia despite the resistance.
"My thoughts and prayers are with the families of our serviceman who was killed and his fellow servicemen who were wounded in Somolia". Its fighters continue to attack the bases of a multinational African Union force that remains largely responsible for security as Somalia's fragile central government tries to recover from decades of chaos.
But it retains a strong presence in regions outside the capital.
The investigation has already triggered changes in the way military activities are carried out in Niger and elsewhere in Africa, including giving teams the option to use heavily armored vehicles and beefed-up firepower.