What is the oxidation state of sulfur in SO3?

The oxidation state of a substance is the electric charge it is exhibiting in a given state. This may be determined by looking at the oxidation states of accompanying atoms as well as the charge on the complete molecule.

In this case:

Molecular charge: 0

Oxidation state of oxygen: - 2 (it is a group 6 element)

Thus,

S + 3 * - 2 = 0

S = 6

Sulfur is exhibiting an oxidation state of + 6.