What was found
AIRCRAFT WAS GROUNDED: PILOT REPORTS AFTER EXTENDING LANDING GEAR, MAIN GEAR PANEL SHOWED 3 RED LIGHTS, WHILE MAIN GEAR AND OVERHEAD GEAR PANEL SHOWED 6 GREEN LIGHTS. ALL GEAR INDICATOR LIGHTS WERE ILLUMINATED. WE DID A GO-AROUND, RETRACTING THE GEAR. WHEN WE EXTENDED THE GEAR FOR A LANDING AFTER REVIEWING QRH PROCEDURES, ALL GEAR INDICATIONS NORMAL. LANDED NORMALLY. MAINTENANCE REMOVED AND REPLACED MASTER TEST RELAY R33. OPS CHECK NORMAL IAW AMM 33-18-00.