The soldiers from the Royal Fusiliers regiment were charged on Tuesday in the UK with mistreating civilians at a food depot, known as the Bread Basket, outside the southern Iraqi city of Basra on or around 15 May 2003.
The troops had been sent to stop looters who were stealing food aid from a warehouse.
The bulk of the evidence in the case is a package of around 22 photographs taken by servicemen at the food depot. One photograph shows two Iraqi men being forced to simulate sex acts while another depicts a man tied to a forklift truck.
Lance Corporal Darren Larkin pleaded guilty to battery for assaulting an unknown male but denied a separate charge of disgraceful conduct of an indecent kind.
"He is ashamed of his unacceptable act," his lawyer William England told the seven-member military panel sitting at Roberts Barracks.
"He realises he has brought shame on his family and his regiment."
Larkin, Corporal Daniel Kenyon and Lance Corporal Mark Cooley face a total of nine charges. Both have pleaded not guilty to all charges.
Judge Advocate Michael Hunter ordered the photographs to be released to the media on the condition that the faces of the civilians and soldiers not charged in the case be obscured to protect their identity.
One photograph shows two Iraqi males being forced to simulate anal and oral sex while another shows an Iraqi tied on to the forks of a forklift truck being driven by one of the accused.
British soldiers are deployed in
the relatively peaceful south
Other photographs show an Iraqi man blindfolded and bound by a cargo net lying on his side on the ground while a soldier simulates punching and kicking.
"It cannot be said that what those photographs depict is anything other than shocking and appalling," the chief prosecutor, Lieutenant-Colonel Nick Clapham, said.
But he urged the military panel hearing the case to assess the photographs "with an attitude of clinical objectivity".
The court martial is expected to last for three to four weeks and is open to the public. The verdicts are subject to appeal.