In a photograph distributed on Twitter by the Spanish coast guard on Monday, the three stowaways are shown crouching on the rudder under the hull, just above the waterline of the Alithini II.
A Canary Islands police spokesperson said it was up to the ship's operator to take care of stowaways, provide them with temporary accommodation and return them to their origin as soon as possible.
However, the migrants may be able to remain in Spain if they claim asylum, Helena Maleno, director of migration non-governmental organisation Walking Borders, told Reuters.
"On several previous occasions, stowaways were able to remain in Spain with political asylum," Maleno said.
The 183-metre ship, sailing under a Maltese flag, arrived in Las Palmas in Gran Canaria after setting out from Lagos in Nigeria on Nov. 17 and navigating up the West African coast, according to Marine Traffic.
Alithini II, which is owned by Gardenia Shiptrade SA, is managed by Athens-based Astra Ship Management, according to public shipping database Equasis.
The ship's captain confirmed to the Red Cross that the ship had sailed from Nigeria 11 days earlier.