"There is also evidence that some soldiers removed body parts from some of the victims," it read, adding that the alleged offences "were executed in January, February and May 2010."
If convicted of any of the three murder charges Gibbs faces life imprisonment.
Three of the other "kill-team" members have agreed to testify against Gibbs: Specialist Jeremy Morlock, Specialist Adam Winfield, and Specialist Michael Wagnon. Another member, Private First Class Andrew Holmes, is not on the witness list.
Gibbs, 26, is also charged with beating up another soldier, Justin Stoner, who reported that unit soldiers were smoking hashish. This triggered the investigation that eventually uncovered the thrill killings.
Gibbs, flanked by defence lawyer Philip Stackhouse, looked on as a five-person panel of three officers and two non-commissioned officers was chosen.
Opening arguments are scheduled to be heard on Monday.
Gibbs is the fourth "kill team" member to face a court-martial for his alleged actions in the southern Kandahar province of Afghanistan between January and May of 2010.
In March Morlock, 23, pled guilty to the killings and to using stolen weapons to pretend the victims were enemy fighters. He was jailed for 24 years and dishonourably discharged, but avoided life in prison.
In August, Winfield, 23, was sentenced to three years in prison after pleading guilty to a reduced charge of involuntary manslaughter and illegal use of marijuana.
Then, last month, Holmes, 21, was jailed for seven years in a plea agreement to avoid a 15-year term. A fifth soldier, Wagnon, is still awaiting military trial for his alleged role in the killings.
The German news magazine Der Spiegel published three grisly pictures in March, out of a reported cache of 4,000 documents, showing Morlock and Holmes holding up the bloodied head of a corpse.
At a pre-trial hearing in November 2010 for Gibbs, defence lawyer Stackhouse argued that his client had played no role in the killings.
Stackhouse asked the judge presiding over the hearing to weigh Gibbs' assertion of innocence with the testimony from "these dope-smoking soldiers in a combat zone. Who are you going to believe; where does the credibility lay?"
Before the court martials got under way the Pentagon admitted that the allegations - and photographs - were embarrassing for the US military, seven years after the Abu Ghraib prison abuse in Iraq.
"It does not help the perceptions of our forces around the world," a Pentagon spokesman said at the time, adding that the allegations were "an aberration" for a force of nearly 100,000 deployed in Afghanistan.
- AAP
