The incident occurred on Friday at Arrabassada Beach. A group of six boys jumped into the sea from the cliffs despite a yellow flag warning of strong waves.
Three of the boys managed to get back to shore on their own, while the others were picked up by rescue personnel.
A 12-year-old boy died at the scene despite resuscitation attempts. Two 13-year-old boys were taken to hospital in critical condition but later died.
Catalonia's regional president, Salvador Illa, expressed his deep sorrow over the deaths and wrote on X that "There are no words in such a moment of sorrow."
The city of Tarragona has declared three days of official mourning.
