What was found
DURING TAXI OUT, NR 3 GENERATOR OFF LIGHT ILLUMINATED NR 3 GENERATOR WAS RESET PER FCOM. DURING CLIMB APPROXIMATELY 15,000 FEET NR 3 GENERATOR VOLTS AND FREQUENCIES WENT TO ZERO. ATTEMPTED RESET NR 3 GENERATOR MAINTAINED VOLTS AND FREQUENCIES, LOAD FOR SECOND THEN WENT TO ZERO. APU WAS USED TO POWER NR 1 AND 3 BUSES FOR SINGLE GENERATOR OPERATION. AIRCRAFT RETURNED TO CCS FOR GENERATOR REPAIRS NR 1 GENERATOR SYSTEM WAS ON DMI. RETURNED AND REPLACED NR 3 GENERATOR CONTROL UNIT PERFORMED ENGINE RUN UP AND SYSTEM OPS CHECKED.