For the uninitiated:
IATA (International Air Transport Association) airline codes have two letters (or a number & letter) whilst ICAO (International Civil Aviation Organisation) codes have three letters. BA needed a third letter for its ICAO code so BAW made sense. (BAL was Britannia.)
With airports IATA have three letters & ICAO four. Heathrow for example is LHR & EGLL The pilots on PPrune tend to use ICAO codes & SLFs like me IATA ones but you can always check them on the web.