At least 25 people are believed to have died when a migrant boat crossed from Mauritania to the Canary Islands.
According to survivors, the bodies were thrown overboard during the crossing, the Spanish police said on Thursday.
The boat arrived on Sunday with 48 people on board on the Canary Island of El Hierro.
The water supplies on the boat ran out within a few days, the police statement said.
Some of the inmates then drank salt water to quench their thirst.
The boatmen are also said to have beaten and tied up a man.
The three suspected boatmen were arrested.
According to the Spanish Interior Ministry, more than 5,500 migrants arrived in the Canary Islands between January 1 and March 15, almost twice as many as in the same period last year.
The rescue ship "Sea-Eye 4" rescued 74 people, including 22 children, from distress at sea on the Mediterranean Sea on Wednesday. The migrants and refugees came from Egypt, Nigeria, Sudan, South Sudan and Syria, the German aid organization Sea-Eye announced on Thursday A total of 106 rescued people are now on board the ship after another rescue operation on Tuesday.
The crew is now looking for a safe place for the people and will reach Maltese waters on Friday.
Another dinghy with 145 people in distress was illegally returned by the Libyan coast guard before the "Sea-Eye 4" could reach them, the aid organization reported, citing the Alarm Phone emergency call service.