What was found
SEA - ON ILS CAT III RUNWAY 3 GEG, AIII STBY WINDOW AT 10 MI FINAL. WHEN ALT SWITCHING TO AIII, NO AIII, NO AIII MESSAGE. ATTEMPTED APPR RESET, NO AIII. SECOND APPROACH SAME RESULTS. ABORTED APPROACH AND DIVERTED BACK TO SEA. REMOVED AND REPLACED THE HGS COMPUTER AS A PRECAUTIONARY MEASURE IAW MTC 6343501, HGS OP'S TESTED GOOD. ACFT AND PLACARD ARE CAT III QUALIFIED, MEL PLACARDS REMOVED FROM ACFT AND LOG BOOK. RETURNED TO SERVICE.