The two men and one woman detained in the Algeciras area about 70 km (44 miles) southeast of Cadiz, are accused of forcing the migrants to jump at knife point into the swell with strong currents, despite knowing that many migrants could not swim.
The trio are also suspected of belonging to a criminal organisation, offences against the rights of foreign nationals, injuries and smuggling.
Images published on social media in November showed a black inflatable speedboat circling in heavy tides off the beach and people on board pushing others off the side.
Police, who made the arrests after finding the boat, said there were at least 37 migrants on board who had paid between 3,000 euros and 12,000 euros ($3,270-$13,075) each for the journey between Kenitra in Morocco and Cadiz.
Despite its proximity to Morocco, the Cadiz area is not a common destination for migrants due to rough seas on the Atlantic coast and surveillance around the Strait of Gibraltar nearby.
The number of migrants arriving in Spain by boat in the first two months of this year quadrupled to 13,485 from the same period a year ago, interior ministry data shows.
Rights group Walking Borders said 6,618 people lost their lives during risky sea voyages trying to reach Spain in 2023.
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