To provide a motor drive control device, an electric power steering device, and a vehicle which can individually diagnose abnormalities of magnetic detection elements, designed in a multisystem configuration to include at least two systems, for each system. A motor drive control device includes two systems of first and second rotation information detection function units. The first and second rotation information detection function units include first and second rotation position information detection units and first and second rotation information detection units. The first and second rotation information detection units individually diagnose their own abnormalities based on first and second motor rotation position signals detected by the first and second rotation position information detection units.