What is the oxidation number for SO3?

When calculating oxidation numbers, use '-2' (oxygen) as the 'yardstick'(standard).

Since SO3 has three oxygens, then the oxygen moiety is 3 x -2 = -6

Since SO3 is a neutrally charged molecule, overall oxidation is '0'.

Hence

S + -6 = 0

Hence S = 6 is the oxudation number for sulphur.

NB Sulphur exhibits different oxidation states in different molecules.

S = 0

SO2 = 4

SO3 = 6 ( as above)

H2SO4) = 6

H2S2O7 (Oleum) = 6