A three finger excercise I learnt as a child was to play individual notes, start with middle finger on c, then ring finger on d, finally little finger on e. Then up a tone, middle on d, ring finger e, little finger f, and so on. Then the reverse starting at c1, little finger on that, then ring finger on b natural, middle finger on a, and so on descending. Very strenuous at first keeping those notes clear and distinct, but improves ring and little finger strength and dexterity.