Here's why:

* Co represents the element cobalt.

* I represents the element iodine.

* The subscript 3 indicates that there are three iodine atoms for every cobalt atom.

* Cobalt(III) indicates that the cobalt ion has a +3 charge (Co³⁺). This is necessary to balance the -3 charge from the three iodide ions (3 x I⁻ = -3).