Two men stand accused of belonging to the jihadist cell that orchestrated the attacks in the Barcelona area in August 2017, while the third defendant is accused of collaborating.
Three men are facing trial in Spain on suspicion of aiding the terrorist cell responsible for the 2017 Barcelona terror attacks.
Legal proceedings begin on Tuesday at the National High Court in San Fernando de Henares, Madrid under heavy police surveillance.
16 people were killed and 140 injured in the attacks on 17 and 18 August 2017, which were claimed by Islamic State.
A rented white van drove into pedestrians on the famous Las Ramblas avenue in the centre of Barcelona, before a similar incident one day later in the seaside resort of Cambrils.
Two of the defendants are alleged members of the cell which planned the attacks, while the third suspect is on trial for being an accomplice of the group.
The main 23-year old defendant in the trial is accused of belonging to a terrorist organisation, manufacturing and possessing explosives, and conspiracy to cause chaos.
He had revealed to investigators that the group's initial plan was to carry out bomb attacks against prestigious sites such as the famous Sagrada Familia basilica, the Camp Nou stadium and even the Eiffel Tower in Paris.
The second defendant, a 31-year-old man, is the brother of one of the perpetrators of the attacks and had rented the van used on the Ramblas.
The third 27-year-old suspect is accused of having lent a vehicle and documents to the assailants.
The public prosecutor's office in Spain has requested prison sentences of 41 years, 36 years and 8 eights respectively for the defendants.
However, the trio is not being prosecuted for the attacks themselves, contrary to the wishes of the civilian parties.
The trial is expected to last until 16 December.
What happened in the 2017 attacks?
On 17 August 2017, 14 people, mostly foreign tourists, were killed and more than 100 hundred injured on Las Ramblas.
In his escape, the 22-year-old Moroccan driver murdered another person to steal their car before leaving the area on foot.
Hours later, five other members of the jihadist cell carried out a second attack 100 kilometres away at the promenade in Cambrils, knocking down several pedestrians and stabbing a woman to death.
The six perpetrators were all later killed by police in a vineyard near Barcelona a few days later.
The imam suspected of having indoctrinated and recruited people for the terrorist cell had been killed in a blast at a villa in Alcanar, caused by stored explosives.
The explosion is believed to have upset the group's initial plans, forcing it to improvise the attack on Barcelona and Cambrils.
The 16 victims of the double attack came from several countries, including Spain, Belgium, Germany, Italy and Portugal.