The trio's ringleader denied any wrongdoing and claimed active US government support at their court appearance on Wednesday.
Jonathan Idema, Brett Bennett and Edward Caraballo were arrested when Afghan security forces raided a house in Kabul on 5 July. American and Afghan officials say they were vigilantes posing as US special forces and had no official backing.
Appearing before a three-judge panel in a national security court, the three listened quietly to the charges including hostage-taking and torture, as three of their ex-detainees described how they were beaten, doused with boiling water and deprived of food.
The Americans did not testify, but Idema said afterwards the abuse allegations were invented. He also said he was in regular phone and email contact with Pentagon officials "at the highest level".
Pentagon contact named
Speaking to reporters crowding round the dock, Idema named a Pentagon official who allegedly asked the group to go "under contract" - an offer they refused.
"The American authorities absolutely condoned what we did, they absolutely supported what we did," he said.
The trial comes at an awkward time for United States officials trying to contain a widening scandal about abuse in US military prisons in Afghanistan and Iraq.
A representative from the US embassy observed the trial, but declined to comment on the proceedings, where only one of the defendants was represented by a lawyer.
Presiding Judge Abd Al-Basit Bakhtyari adjourned the case for at least two weeks to give the Americans and four Afghans accused of helping them more time to prepare their defence.