SO2canact as both oxidizing as well as reducing agent since it has +4 oxidation state which is exactly between its highest oxidation state +6 and lowest oxidation state +2 ,Thusit can change its oxidation number either from +4 to-2(reduction) and +4 to+6 (oxidation) .

A reducing agent will reduce, and thereby get further oxidized in a redox reaction. If SO3 acts as a reducing agent, then Sulfur in SO3 should achieve an oxidation state higher than what it has in SO3 (+6).Oxidation state of sulfur in SO3 is VI (+6). This means that 6 electrons have already been stripped from the valence shell of sulfur. Electronic configuration of sulfur is 1s2, 2s2p6, 3s2p4. So, the entire complement of 6 electrons from the 3rd shell (3s2p4) have been lost from the sulfur in SO3. In order for SO3 to act as a reducing agent, it has to come up with one or more electrons to donate to the substrate. It will have to do so from its inner shells - 1s2, 2s2p6.We do not have a single example of electrons from a completed inner shell of an atom with any of the elements in the main group chemistry. So, it is highly unlikely that this will happen in the case of S(VI) in SO3.That is why SO3 can only act as an oxidizing agent (and not as a reducing agent), and happily accept one or more electrons.

tq so much