The 3 disc headcode indicates a train on the Western section of these routes:-
West London line to Brighton
Windsor and Dover (Excursion trains) as between Redhill and Dover
Waterloo and Portsmouth Harbour via Eastleigh
Feltham and Brent via Richmond
detail courtesy of
http://www.semgonline.com/headcodes/sheadcodes/04.html CLIVE HANLEY(C)