French President Francois Hollande is awarding the trio the country's top honour, the Legion d'Honneur, for their bravery.
It comes as more details emerge about the background of the alleged gunman, and as questions are asked about apparent security failures.
Police allege the gunman boarded the high-speed train to France in Brussels armed with a Kalashnikov, an automatic pistol, nine cartridge clips and a box-cutter.
The attacker wounded three people before being overpowered by passengers, who included two members of the United States armed forces.
Off-duty US airman Spencer Stone used his fingers to plug a gunshot wound in the neck of a fellow passenger.
He says he and his friends, Anthony Sadler and Alek Skarlatos, had to act quickly to restrain the gunman.
"I turned around and I saw he had what looked to be an AK-47 and he looked like it was jammed and wasn't working and he was trying to charge the weapon and Alek just hit me on the shoulder and said 'Let's go' and ran down, tackled him, we hit the ground, Alek came up and grabbed the gun out of his hand while I put him in a choke-hold."
His civilian friend, Anthony Sadler, says they couldn't just sit back and do nothing.
"Hiding or sitting back is not going to accomplish anything. The gunman would have been successful if my friend Spencer had not gotten up, so I just want that lesson to be learned going forward: in times of terror like that to please do something, don't just stand by and watch."
A French citizen who also tackled the attacker wished to remain anonymous.
US Ambassador to France Jane Hartley says the men's actions may have prevented a massacre.
"We often use the term hero and in this case I think that word has never been more appropriate. I know these young men sitting with me won't like it because even in the brief period we've known each other, they are so humble but they are truly heroes."
French authorities say the suspect, 25 year-old Moroccan national Ayoub El-Kahzzani, has links to radical Islamism.
He is reported to have lived in France, Spain and Belgium and to have travelled to Syria.
Mr El-Khazzani's lawyer, Sophie David, says he is "dumbfounded" by the terror allegations and claims he only meant to rob people.
"He says nothing happened. When I tell him that it is all over the news he doesn't understand that because for him nothing happened. He didn't have time to do anything, he was disarmed immediately.".
Questions are now being asked about Europe's security preparedness, and about how a man flagged by counterterrorism authorities in a three countries as a potential risk was allowed on the high-speed train without any security checks.
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