ISO 3166-1 alpha-3 three-letter country code (e.g., 'USA', 'NGA', 'COL'). Best-effort enrichment derived by mapping COW numeric codes to ISO3 via a static lookup table. NOT authoritative — intended only as a convenience join key for PSED-era datasets (Africa, Latin America, Southeast Asia, 1990–2012). Countries outside that scope may have NULL or incorrect ISO3 values.