Catalonia's regional police identified them as Moussa Oukabir, 17, Said Aallaa, 18, and Mohamed Hychami, 24. Police said they were searching for a fourth suspect, Younes Abouyaaqoub, aged 22.
Police on Friday stepped up their investigation into the twin vehicle attacks in Spain that left 14 dead and over 100 more injured in a bustling tourist area of Barcelona and the nearby seaside resort of Cambrils.
The attacks claimed victims and wounded from three dozen countries.
Of the 12 people police suspect of involvement in the attacks, five were shot dead by security forces in Cambrils and another four have been arrested, said Josep Lluis Trapero of Catalonia's police during a television interview late Friday.
The three remaining suspects have been identified but have not been detained, he added.
Police suspect two of them may have died in a blast at a house in Alcanar, about 200 kilometres (140 miles) south of Barcelona, where the group is believed to have been preparing explosive devices.
Officers have found "the remains of two different people, we are working to prove that they are two of these three people who have been identified," said Trapero.
Police have not yet identified who drove the white van that sped into crowds on the busy Las Ramblas avenue in central Barcelona, leaving 13 people there dead, he added.
Earlier on Friday Trapero said the group was preparing "one or several attacks in Barcelona" with explosive devices but after the blast at the house in Alcanar they moved quickly to commit "more rudimentary" attacks.