What was found
1.5 HOURS INTO FLIGHT, AT FL350, RECEIVED A MESSAGE FOR NAV ADR 2 & 3 FAULT. RECEIVED AN AUTO FLT AP OFF /AUTO FLT, A/THR OFF / F/CTL ALTN LAW. FLIGHT DECLARED AN EMERGENCY, RETURNED TO DEPARTURE, AND LANDED WITHOUT FURTHER INCIDENT. THE AIRCRAFT WAS GROUNDED. REPLACED AIR DATA INERTIAL REFERENCE UNIT NR 2 AND 3 IAW AMM 34-12-34. OPS CHECKED GOOD. ALSO PERFORMED AN OPS TESTED IAW AMM 34-10-00-860-801-A, 34-10-00-860-802-A, AND 34-10-00860-803-A. ALL TESTED GOOD.