The Islamic State group took responsibility for the August 17-18th bloodshed, including a van ramming passers-by, all part of a string of jihadist attacks across Europe.
Although the six perpetrators were shot dead by police, many questions remain with victims and prosecutors hoping Spain's National Court trial will shed light on how the violence unfolded.
Some 235 witnesses will be called during the trial at a court near Madrid which will run until December 16.
While none of the trio are charged with direct responsibility, they are in the dock for helping the attackers, one of whom ran down pedestrians on Barcelona's Las Ramblas on a busy Saturday, killing 14.
The driver fled and killed another person before being shot dead several days later.
In the early hours of Sunday morning, five others rammed pedestrians in Cambrils, 100 kilometres (60 miles) further south, and fatally stabbed a woman before being shot dead by police.
Main defendant Mohamed Houli Chemlal refused to answer questions in court about the facts, pointing to statements given days after the attacks.
He said he was willing to cooperate and asked if he was sorry replied: “Obviously.”
Prosecutors then showed footage of the cell preparing an explosive belt and saying the explosives would do “a lot of damage”.
Driss Oukabir, 31, a brother of one of the attackers who is accused of renting the Barcelona van, denied being part of a terror cell, insisting he “wasn't religious”.
He said his brother and 22-year-old Younes Abouyaaqoub, the Barcelona driver, had asked if he would hire a van as a favour “for moving house, because they weren't old enough to rent one”.
Said Ben Iazza, 27, admitted lending his ID card and vehicle to two business customers who said they were going to buy cleaning products.
He denied knowing they wanted to fabricate explosives and, like Oukabir, insisted he didn't know Abdelbaki Es Satty, the 44-year-old imam who radicalised the young men in Ripoll, their hometown in Pyrenean foothills.
Chemlal and Oukabir are accused of belonging to a jihadist group, manufacturing and possessing explosives and conspiracy to wreak havoc, while Ben Iazza is charged with collaborating with the group.
Prosecutors say they want Chemlal jailed for 41 years, Oukabir for 36 years while Ben Iazza faces an eight years.
During the investigation, Chemlal admitted the initial plan was to hit targets like Barcelona's Sagrada Familia basilica, with police finding documents relating to the city's Camp Nou football stadium and the Eiffel Tower.
Taking the stand, an unidentified investigator said evidence was found that August 20th would be the date they “very likely planned to carry out a terrorist attack” with vans, explosive vests and grenades.
The likely target would be Camp Nou during a match with a focus on the exits, he said.
However, on Friday, August 15th, an accidental explosion at a house in Alcanar between Barcelona and Valencia where they had been preparing explosives, pushed them to hurriedly improvise the weekend attacks.
Chemlal survived the blast, but it killed the imam, described by the investigator as the “spiritual leader” who would send orders to his most-trusted inner circle which included three of the attackers.
Chemlal was in the second circle of trust, Driss Oukabir in the third, meaning he “took part but not in such a direct way” as the others.
Cell members were linked by close friendship and family ties and didn't interact with outsiders.
“They had all known each other since they were small, making the group practically impenetrable,” the investigator testified. “The secrecy between them was extraordinary.”
Antonio Guerrero, a lawyer for the AVT terror victims' association said all three should be tried for “terrorist murder”.
“In our opinion, they were part of the cell,” he said.
Lawyer Jaume Alonso-Cuevas said there were “too many unanswered questions”.
“We want the state to be accountable for not regulating the sale of explosives purchased by the cell to prepare for attacks.”
By Alvaro Villalobos and Daniel Silva