But he insisted he wasn't involved in the first or third killings, and in the second he merely returned fire.
Prosecutors said Gibbs and his co-defendants knew the victims posed no danger, but dropped weapons by their dead bodies to make them appear to have been combatants.
Video: U.S. Sgt. allegedly orchestrated "kill team" of troops to murder civilians in Afghanistan
Alleged "Thrill Kill" Soldier's Hearing Begins
Three of the co-defendants pleaded guilty, and two of them testified against him, portraying him as an imposing, bloodthirsty leader. Gibbs' lawyer insisted they conspired to blame him for what they had done and told the five jurors the case represented "the ultimate betrayal of an infantryman."
The jury deliberated for about four hours before convicting him. He faces, at minimum, life with parole, and at maximum life withoutit.
The investigation into the 5th Stryker Brigade unit exposed widespread misconduct -- a platoon that was "out of control," in the words of a prosecutor, Maj. Robert Stelle. The wrongdoing included hash-smoking, the collection of illicit weapons, the mutilation and photography of Afghan remains, and the gang-beating of a soldier who reported the drug use.
In all, 12 soldiers were charged; all but 2 have now been convicted.
The probe also raised questions about the brigade's permissive leadership culture and the Army's mechanisms for reporting misconduct.
After the first killing, one soldier, then-Spc. Adam Winfield, alerted his parents and told them more killings were planned, but his father's call to a sergeant at Lewis-McChord relaying the warning went unheeded. Winfield later pleaded guilty to involuntary manslaughter in the last killing, saying he took part because he believed Gibbs would kill him if he didn't.
The case against Gibbs relied heavily on testimony from former Spc. Jeremy Morlock, of Wasilla, Alaska, who is serving 24 years after admitting his involvement in all three killings.
According to Morlock, Gibbs gave him an "off-the-books" grenade to use in the first killing -- a teenager in a field -- in January 2010; killed the second victim and tossed an AK-47 at his feet to make him appear to have been an enemy fighter the next month; and threw a grenade at the third victim, in May, as he ordered Morlock and Winfield to shoot.
Morlock and others told investigators that soon after Gibbs joined the unit in 2010, he began talking about how easy it would be to kill civilians, and discussed scenarios where they might carry out such murders.
Asked why soldiers might have agreed to go along with it, Morlock testified that the brigade had trained for deployment to Iraq before having their orders shifted at the last minute to Afghanistan.
The infantrymen wanted action and firefights, he testified, but instead they found themselves carrying out a more humanitarian counter-insurgency strategy that involved meetings and handshaking.
Another soldier, Staff Sgt. Robert Stevens, who at the time was a close friend of Gibbs, told investigators that in March 2010, he and others followed orders from Gibbs to fire on two unarmed farmers in a field; no one was injured. Gibbs claimed one was carrying a rocket-propelled grenade launcher, but that was obviously false, Stevens said.
Stevens also testified that Gibbs bragged to him about the second killing, admitting he planted an AK-47 on the victim's body because he suspected the man on involvement with the Taliban, according to a report on the testimony in The News Tribune newspaper of Tacoma.
But during the trial, Gibbs insisted he came under fire.
"I was engaged by an enemy combatant," he said. "Luckily his weapon appeared to malfunction and I didn't die."
Gibbs testified that he wasn't proud about having removed fingers from the bodies of the victims, but said he tried to disassociate the corpses from the humans they had been as a means of coming to terms with the things soldiers are asked to do in battle.
The muscular 6-foot-4 staff sergeant also testified that he did it because other soldiers wanted the trophies, and he agreed in part because he didn't want his subordinates to think he was a wimp.
Gibbs initially faced 16 charges, but one was dropped during the trial.
'Kill team' leader Calvin Gibbs guilty of murder
'Kill team' leader Calvin Gibbs guilty of murder
The 26-year-old admitted to cutting and keeping fingers from corpses as war trophies, but said he was returning enemy fire and did not set out to kill.
Three of the co-defendants in the case pleaded guilty, and two testified against him.
Gibbs, from Billings, Montana, was the highest-ranking of the soldiers charged with murder.
Prosecutors told the jury that Gibbs and the other soldiers dropped weapons by the bodies to make them appear to be combatants.
Gibbs' lawyer argued that the three who pleaded guilty conspired to blame him for their own actions.
The jury of five deliberated for four hours before announcing the verdict, pronouncing him guilty on all 15 charges against him.
He faces life in prison, either with or without parole.
Unanswered call
The investigation initially began as a wider inquiry into 5th Stryker Brigade, the unit Gibbs deployed with to Afghanistan.
The platoon was "out of control", a prosecutor said, including smoking hash, photographing Afghan remains and beating a soldier who reported the drug use.
One member of the brigade, then-Spc. Adam Winfield, told his parents when the first Afghan civilian was killed, and alerted them more killings were planned.
But nothing appeared to change after Winfield's father called to report the information to a sergeant at the American base where the brigade was based.
Winfield pleaded guilty to involuntary manslaughter in the last killing, and received a reduced sentence.
He testified at a hearing that he believed Gibbs would kill him if he did not take part in the violence.
Testimony also came from Specialist Jeremy Morlock, who is serving 24 years in prison for his involvement.
According to Morlock, when Gibbs joined the platoon in 2010, he began describing scenarios to kill civilians, saying it would be easy to get away with.
Morlock testified that Gibbs used grenades against two of his victims, and placed an AK-47 near another victim to make it appear he had been armed.
'Disassociated'
Gibbs took the stand in his own defence during the seven-day court martial, saying he kept fingers off the killed Afghans because he was "disassociated" during combat and said it was "like keeping the antlers off a deer you'd shoot".
The killings occurred during routine patrols in Afghanistan's Kandahar province in early 2010.
In March 2011, photographs were published showing the soldiers posing with the bloody corpses of Afghan civilians they had just killed.
The images, described by the US Army as "disturbing and in striking contrast to the standards and values of the US Army" prompted the army to apologise for the distress the photos caused.
In addition to the five soldiers charged with murder, seven other soldiers were charged with less serious, related offences.
Most have agreed to plea deals and all but two have been convicted.
