I am assuming you mean Co_{3}O_{4}.

The only way I can think of is by trial and error. Neither of the three unknowns (a, b, c) is higher than 10 (most likely they are lower), they are all integers. See if any combination yields mass of the original sample that is close to 0.3283 g.

4.5 yields quite a good approximation. (While 4.5 is not an integer, case is covered by a=2, b=2, c=9).