After the one-day hearing ended judges announced they would make a decision in full next month.
Colonel Chhouk Rin, looking thin and unhealthy, told reporters in the capital Phnom Penh he was unsure what the judges would decide, as he was whisked into the Appeals Court amid tight security on Monday.
"We will wait and see what the decision of the court will be," he said.
Australian David Wilson, 29, Briton Mark Slater, 28, and French national Jean-Michel Braquet, 27, were abducted from a train travelling between Phnom Penh and the southern city of Sihanoukville in 1994.
Thirteen Cambodians also died in the attack and the Westerners were held for two months before ransom negotiations failed. They were killed by the rebels when government forces attacked their hideout.
In September 2002, a Cambodian court reversed an earlier decision and sentenced Chhouk Rin in absentia to life imprisonment for the abduction-murder after he refused to appear in the court case.
He remained in his remote base at Vine Mountain in southeast Cambodia, while his lawyers pushed for Monday's retrial after successfully arguing there had not been enough time to prepare their client's case.
Lawyers for Chhouk Rin said they would call 24 witnesses.
Khmer Rouge general Nuon Paet and a senior commander Sam Bith have also been sentenced to life in prison for their part in the killings.