The official with the Superior Court of Catalonia says the arrests were made Tuesday in Tarragona province.
The suspects will go before a judge Thursday. The official said she had no other details. She spoke on condition of anonymity in line with court rules.
Spanish newspaper El Pais said the suspects are two Pakistani men and a woman from Romania. They are suspected of having traveled to another European city to collect ransom money, the paper said, quoting police.
The child, Sahil Saeed, was released Tuesday, almost two weeks after being abducted from his grandparents' house in central Pakistan. A senior Pakistani provincial official said a ransom was paid for the release of the child, who is of Pakistani origin.
Spanish police who raided the apartment where the detainees had been living for at least a year seized an undisclosed sum of money, El Pais added. Tarragona is part of the Catalonia region of which Barcelona is the capital.
Spanish authorities worked in conjunction with British and Pakistani counterparts.
Pakistani authorities have said someone in the family was involved in the abduction - a charge the family denies.
