The definition of a Lewis acid and base is: Acid is any substance that can accept a pair of nonbonding electrons, while that a Base is a species that contains a nonbonding pair of electrons, and in the reaction in the question:
SO3 + H2O -> H2SO3 + 2 OH-
We have SO3 as the conjugate base of the acid H2SO3 and H2O is acting as an acid in this occasion, therefore the answer is Acid