My music teacher has always told me to use only pinky, middle finger and thumb (5-3-1) while practicing the triad chords. Though I am trying to use it, it becomes difficult on the black keys. Personally, I find playing the triads with left hand using 4-2-1 much simpler than 5-3-1. but since the teacher keeps telling me to use the 5-3-1 (even on black keys) because (as per him) it is recommended by the music examination bodies, I am really wondering if that is really true. Do the music examination bodies like trinity mandate playing the triads with with only (5-3-1) in Left Hand and (1-3-5) in Right Hand? If yes, why? If I find it really comfortable playing it with 4-2-1, why should I use 5-3-1?
Related question: In the above context, what fingering to use for triads on black keys?
Note: There is one question already on the forum with the title "Is it right to play a triad left hand chord with fingers 4-2-1 on a piano?", but that did not satisfy my question correctly.