In any case, John Campbell was promoted lieutenant-colonel, commission to be dated 18th of June, 1815, for his conduct during the battle of Waterloo. So most probably he was in command.
He was also appointed Companion of the Most Honourable Military Order of the Bath for his 'deeds'.
Commanding officer of 3 Line Bat. K.G.L. was Lt-Col Fred. de Wissell, although the senior officer was Colonel H.G. de Hinuber.
Best regards, Geert