Rogelio Leyva, 19, was taken into custody just before noon in Alpine, said San Diego police homicide Lt. Terry McManus.
He, Daniel Flores, 18, who was arrested Saturday in San Diego, and Flores' brother, 20-year-old David Flores, are suspects in the death of Martin Omar De La Torre, 18, of San Diego.
De La Torre was shot in the head Monday evening in the 800 block of South 30th Street, McManus said.
David Flores was arrested Tuesday, McManus said. He is charged with murder and shooting into an occupied vehicle, and faces life in prison if convicted, according to Deputy District Attorney Frank Jackson.
Authorities believe the three were involved in a car-to-car shooting about an hour before De La Torre was shot, and that David Flores was driving a car April 1 when passengers shot at another car.