What was found
(CAN) SHORTLY AFTER TAKEOFF, FLIGHT CREW UNABLE TO ENGAGE AUTO-PILOT. FLIGHT HAD DEPARTED WITH 1 INOPERATIVE ON MEL, BUT APPARENT FAILURE OF ALL 3 OCCURRED. WORKED WITH MOC FOR POSSIBLE RE-SETS, BUT UNABLE. RETURNING TO STATION, AND IS EXPECTED TO LAND APPROXIMATELY 20 TONS OVERWEIGHT. MCDP BITE CHK C/OUT TEST 30 CURRENT FAULT REPORTS, FAULTED FCC "C". C/O TEST 40 A/L SERVO TEST, FAULTED FCC "C". REPLACED FCC "C" IAW AMM 22-11-01 AND GROUND TEST 01 FCC C/OUT SERV. TEST 40 C/OUT AGAIN, THIS TIME DID NOT FAULT FCC "C". MCDP TEST 40 C/OUT SERV. AFDS OPS TEST C/OUT SERV IAW AMM 22-10-00. NO FAULTS.