What was found
IN CRUISE FLIGHT AT FL310 HYD 3 LO PRESS CAS ILLUMINATED. SYSTEM SHOWED 2% QUANITY AND 0 PSI. EMERGENCY DECLARED AND DIVERTED TO DJT. ACCOMPLISHED HYD 3 LO PRESS QRH AND MANUAL GEAR EXTENSION QRH PROCEDURES. AFTER MANUAL GEAR EXTENSION, A GEAR DISAGREE MESSAGE PRESENTED ON THE EICAS, THERE WERE THREE GREEN/DOWN INDICATIONS ON GEAR. LANDED NORMALLY, STOPPED ON RUNWAY TOWED TO RAMP. CORRECTIVE ACTION: REPLACED O-RING. OPS CHECK GOOD. IAW CL650 AMM 29-14-00