Referring to the only soldier convicted in the gruesome My Lai Massacre of civilians during the Vietnam War, Mr. Solis said: “People think Nixon pardoned Lieutenant Calley, but he didn’t. Calley was paroled.”

Presidents all the way back to George Washington have granted pardons to tens of thousands of American troops, but nearly all were young men who deserted or who evaded a draft, and received clemency after the fighting ended.

While the new pardons are a stark departure from tradition, they are in line with Mr. Trump’s many statements during his campaign and in office, arguing that to beat unconventional enemies like the Taliban and ISIS, the American military should loosen the reins on how troops behave in conflict zones.

“You have to play the game the way they are playing the game,” he told NBC News in 2016.

The specific circumstances of the three men’s cases defy easy characterization. In one, a decorated captain admitted to a killing in a job interview. In the other two, platoon leaders’ illegal actions were reported not by superior officers or Pentagon lawyers, but by their own platoons.

Troops who testified in those two cases, against Lieutenant Lorance and Chief Gallagher, voiced disappointment and disbelief over Mr. Trump’s plans for clemency before they were announced.

“The tragedy of pardoning Lorance isn’t that he will be released from prison — I’ve found room for compassion there,” said Patrick Swanson, a former Army captain who was Lieutenant Lorance’s company commander in Afghanistan. “The tragedy is that people will hail him as a hero, and he is not a hero. He ordered those murders. He lied about them.”

Mr. Lorance was a rookie Army lieutenant who had been in command of a platoon in Afghanistan for two days in July 2012 when he ordered his troops to fire on unarmed villagers who posed no threat, killing two men. He then called in false reports over the radio to cover up what had happened. He was immediately turned in by his own men.