Provided are a vehicle-mounted power source control apparatus, a vehicle-mounted power source apparatus, and a vehicle control method that are highly versatile. A power source control apparatus sets a setting value corresponding to a combination of loads to be connected to multiple output ports based on information determining a candidate value for a setting value for each combination of the loads. Also, control relating to backup is performed based on the set setting value. By doing so, it is possible to perform control relating to backup in a mode corresponding to the combination of the loads.