Three letter abbreviations in shorthand

I work for an airline and was taking down some notes from a booking.  I had to take down three letter city codes and found that it is easiest to do in Teeline shorthand, which is NOT phonetic, but alphabetic.  I had to take down LHR (London Heathrow) and MAA (Madras) and DXB (Dubai).  Now how would you go about taking abbreviations or codes like this in Gregg?  And I mean Gregg, not spelling them out as an exception and not writing the actual city name instead of the code.  Also Teeline is better than Greeg or Pitman for taking down other sometimes unreadable babble like weird website names etc.  Is there some trick for this in Gregg?

(by wordsigner for everyone)