I think the instructions on this page go against the ID3 specification.
From the ID3 2.4 spec:
TIT1 The 'Content group description' frame is used if the sound belongs to a larger category of sounds/music. For example, classical music is often sorted in different musical sections (e.g. "Piano Concerto", "Weather - Hurricane").
TIT2 The 'Title/Songname/Content description' frame is the actual name of the piece (e.g. "Adagio", "Hurricane Donna").
TIT3 The 'Subtitle/Description refinement' frame is used for information directly related to the contents title (e.g. "Op. 16" or "Performed live at Wembley").
If I read that correctly, then the work "Concerto in G major Op. 9 no. 6" should correctly be tagged as follows:
TIT1 │TIT2 │TIT3
Concerto in G major │ I. Allegro │ Op 9 no 6
Concerto in G major │ II. Adagio (non troppo) │ Op 9 no 6
Concerto in G major │ III. Allegro │ Op 9 no 6