What was found
(CAN) DURING TAXI A PASSENGER NOTICED THAT THE RT INBOARD NR 3 TIRE APPEARED TO BE LOW ON PRESSURE. AFTER THE AIRCRAFT RETURNED TO THE GATE, MAINTENANCE FOUND THE NR 3 TIRE AT 60 PSI (NOMINAL IS 141 PSI). THE NR 3 MAIN WHEEL AND THE AXLE MATE NR 4 MAIN WHEEL WERE REPLACED AND THE AIRCRAFT RETURNED TO SERVICE. FURTHER INVESTIGATION OF THE NR 3 TIRE AFTER REMOVAL FOUND THAT THE TIRE WAS LEAKING THROUGH THE SIDEWALL.