US National Guardsman Alek Skarlatos, US airman Spencer Stone and student Anthony Sadler will give evidence in the case against Moroccan Ayoub El Khazzani.
El Khazzani is accused of planning to kill hundreds of train passengers in a gun attack on 21 August 2015.
A month-long trial for the 31-year-old and his three suspected accomplices started yesterday. If convicted of attempted terrorist murder, he will face life imprisonment.
The attempted attack is believed to have been planned from Syria and led by Abdelhamid Abaaoud, who led a series of 2015 assaults on Paris, including the Bataclan Theatre attack.
In the Daesh-inspired attack, El Khazzani boarded a Thalys high-speed train to Paris, in Brussels, carrying a Kalashnikov with nine magazines with 30 rounds of ammunition each.
The 31-year-old was carrying an automatic pistol, a cutter, and a bottle of highly flammable yellow liquid, the judge said yesterday.
The Moroccan, who was bare-chested and may have been on drugs, according to a Metro report, was stopped by the three American tourists, who tackled him to the ground.
The men, all from California, said they had acted on “gut instinct”, according to an Associated Press (AP) report.
Stone, joined by British businessman Chris Norman, reportedly choked El Khazzini until he was unconscious.
The US airman then administered first aid, saving the life of French-American Mark Magoolian who had been shot in an earlier attempt to neutralise El Khazzini.
The train was then re-routed to Arras, in northern France, where El Khazzini was arrested by local police.
The three Americans have been praised for saving the lives of the 150 passengers in El Khazzini’s carriage and for preventing a “slaughter”, the court heard.
The attorney, Thibault de Montbrial, went on to term the Americans’ actions a “very brave intervention”.
“This terror attack could have killed up to 300 people based on the number of ammunition that was found on the terrorist and in his bag,” De Montbrial added.
The three US nationals and Briton Chris Norman were honoured with French citizenship and made members of the Légion d’Honneur, France’s highest award.
The events inspired the movie “The 15:17 To Paris“, directed by Clint Eastwood, which features the three Americans re-enacting the scene as themselves.
Eastwood was on the witness list for the trial, however, the court decided yesterday that because he was not a direct witness to the crime, he does not need to testify.