The shooting at Cure International Hospital in western Kabul was the latest in a string of deadly attacks on foreign civilians in the Afghan capital this year.
Two of the dead Americans were a father and son, Minister of Health Soraya Dalil said. The third American, Chicagoan Jerry Umanos, was a Cure International doctor who had worked in Kabul for seven years. Dalil said an American nurse was also wounded in the attack.
The LCHC says Umanos worked there for 16 years, until 2005 when he and his wife Jan moved to Afghanistan.
"For over a decade, Dr. Jerry Umanos has volunteered in Afghanistan to train medical residents and to see pediatric patients," said Dr. Bruce Rowell of Lawndale Christian Health Center. "This is a great loss for his family, for those of us he worked with as well as for the people of Afghanistan."
"We don't hold any ill will towards Afghanistan in general or even the gunman who did this," said Jan Schuitema, Umanos' wife. "We don't know what his history is."
The attacker was a member of the Afghan Public Protection Force assigned to guard the hospital, according to District Police Chief Hafiz Khan. He said the man's motive was not yet clear.
The gunman, who was detained, was wounded during the attack and underwent surgery at midday in the same medical facility under heavy police guard, according to Kanishka Bektash Torkystani, a Ministry of Health spokesman.
Later in the afternoon, Dalil, the health minister, said he was recovering from the surgery before being questioned. Initial reports indicate he was shot by other security forces, said Ministry of Interior spokesman Sediq Sediqqi.
"Five doctors had entered the compound of the hospital and were walking toward the building when the guard opened fire on them," Torkystani said. "Three foreign doctors were killed."
According to its website, the Cure International Hospital was founded in 2005 by invitation of the Afghan Ministry of Health. It sees 37,000 patients a year, specializing in child and maternity health as well as general surgery. It is affiliated with the Christian charity Cure International, which operates in 29 countries with the motto "curing the sick and proclaiming the kingdom of God."
The Afghan capital has seen a spate of attacks on foreign civilians in 2014, a worrying new trend as the U.S.-led military coalition prepares to withdraw most troops by the end of the year.
It was unclear whether the Taliban were behind Thursday's shooting, though the insurgents have claimed several major attacks that killed foreign civilians this year, an escalation of such attacks after years of mostly targeting foreign military personnel and Afghan security forces.
"We have lost a dear friend, our clinic is grieving right now, our hearts are broken," said James Brooks of the Lawndale Christian Health Center.
Umanos' family acknowledges immense pain from mourning this loss. At the same time, his wife asks those listening to consider what he would want.
"Please honor Jerry's memory by opening up your hearts to the Afghan people," Schuitema said.
"Dr. Jerry Umanus was a proud Chicagoan and an American hero who sacrificed his own life to save others. His senseless killing while on a humanitarian mission in Afghanistan has left a deep wound on our entire city. He was a talented pediatrician, a loving husband, and a respected leader at the Lawndale Community Church who spent more than a decade training fellow doctors and treating some of the world's most needy. He embodied the very best of Chicago and on behalf of the entire city, I send my deepest condolences to his family, friends, and entire community," said Chicago Mayor Rahm Emanuel in a statement.
The Associated Press contributed to this report.