Cobalt (III) has a charge of 3+, and Oxygen has a charge of 2-.
So when you put these two into a compound, you can use the charges of the opposite atom as the number of atoms in the compound. To clear up this statement, let's make this compound:
[tex] Co_{2} O_{3} [/tex]
This formula ensures that the compound is neutral. The cobalt has a +6 overall and the oxygen has a -6 overall, giving a net 0 charge.
This means that there are 2 atoms of Cobalt (III) in the cobalt (III) oxide compound.
