Hunsaker and Private First Class Corey Clagett have already pleaded guilty and co-operated with prosecutors. They were sentenced to 18 years in military prison.
Hunsaker testified on Tuesday that after the squad took the three detainees into custody, Girouard told the soldiers that the group's first sergeant was angry that the detainees were still alive.
Hunsaker said he and Clagett took the three detainees outside, away from other soldiers.
Hunsaker said that he pulled down their blindfolds and looked them in the eyes and that Clagett told them in Arabic to run.
"I shot him [the first detainee] where his heart should be. I moved from right to left. I took aim in the same manner and aimed for the heart and the head,'' Hunsaker said.
He said that after the detainees were shot, Girouard cut Hunsaker with a pocket knife, saying: "It's got to look good."
Another soldier, Specialist Bradley Mason, testified that he heard Girouard order Clagett and Hunsaker to kill the detainees.
Helped 'cover up'
Anita Gorecki, Girouard's lawyer, said in opening statements that the sergeant never ordered his soldiers to shoot the detainees, but that he did help cover up the killings.
"He realised they killed the detainees, and in that moment, yes, he decided to help his squad members," she said.
The soldiers had previously told investigators they were given rules of engagement by Colonel Michael Steele, the 3rd Brigade commander, to kill all military-age men.
Steele has denied this but invoked his right not to testify.