As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SEA - FLIGHT 2039 - ON APPROACH, THE GEAR HANDLE WAS PUT DOWN BUT THE GEAR DID NOT COME DOWN AND THREE RED LIGHTS WERE ILLUMINATED. NORMAL HYDRAULIC PRESSURE AND QUANTITY WERE NOTED. THE GEAR EVENTUALLY CAME DOWN USING THE ALT EXTENSION METHOD. MAINTENANCE REMOVED AND REPLACED THE LANDING GEAR SELECTOR VALVE IAW AMM 32-33-11. OPS CHECKS GOOD.