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.
The 183-metre ship, sailing under a Maltese flag, arrived in Las Palmas in Gran Canaria after setting out from Lagos in Nigeria on November 17 and navigating up the west African coast, according to Marine Traffic.
The ship's captain confirmed to the Red Cross that the ship had sailed from Nigeria 11 days earlier.
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.
Alithini II, which is owned by Gardenia Shiptrade SA, is managed by Athens-based Astra Ship Management, according to public shipping database Equasis.
Astra Ship Management did not immediately respond to calls seeking comment.
The coast guard said the migrants were rescued by a coast guard vessel at about 7pm on Monday.
The stowaways were being treated for moderate dehydration and hypothermia, the Canary Islands emergency services and the Red Cross said.
One of the migrants was in a more serious state and had to be taken to a different hospital on the island.
The Spanish-owned Canary Islands are a popular gateway for African migrants attempting to reach Europe.
The number of migrants arriving illegally to the archipelago by sea fell 17.6 per cent to 14,875 in the first ten months of 2022 from a year earlier, according to the Interior Ministry.
