CAMP PENDLETON, Calif. — A third Marine pleaded guilty to reduced charges yesterday in the killing of an unarmed Iraqi civilian, whose body was left with an AK-47 and a shovel to make him look like an insurgent.

Lance Corporal Jerry Shumate Jr. entered his pleas to aggravated assault and conspiracy to obstruct justice through his civilian attorney, Steve Immel, during court-martial proceedings.

He was expected to testify about the April incident in a deal that called for dismissal of the original charges of murder, kidnapping, conspiracy, larceny, assault, and housebreaking, Mr. Immel said.

Shumate was part of a squad of seven Marines and a sailor who were charged with kidnapping 52-year-old Hashim Ibrahim Awad in the rural Iraqi town of Hamdania, dragging him to a roadside hole, shooting him, and then trying to cover it up.

Two other Marines and the Navy corpsman who was on patrol with the squad earlier pleaded guilty to lesser charges. The longest any of the three was ordered to serve is 21 months.

They testified that the group planned to kidnap and kill a known insurgent and, when they couldn’t get to him, some members of the squad went into a neighboring house and instead kidnapped Awad, whom prosecutors have described as a disabled father of 11 and a former police officer.

Prosecutors say squad members tied up Awad, put him in a hole, and shot him, then placed an AK-47 in his hands and a shovel in the hole to make it appear he was an insurgent planting explosives. The Pentagon began investigating shortly after Awad was killed in April.

Four senior squad members still face kidnapping and murder charges.

Meanwhile, a young Marine who was called back from Iraq to be with his ailing wife just before she died of childbirth complications was arrested on suspicion of murdering his newborn son.

Authorities did not disclose a motive for the slaying, but the Marine, 20-year-old Robert Quiroz, told a TV station in September that he felt overwhelmed as a single parent of two and struggled to accept his son.