Is this a neutralization reaction? H2O + SO3 = H2SO4?

No. A neutralization reaction starts with a base and an acid, ends with the products of water and a salt. H2O is not a base or acid. This reaction is a synthesis reaction, where two simpler molecules combine to make a more complex molecule. The word equation happens to be: water + Sulfate -> Sulfuric Acid. It's balanced, however it is not a neutralization reaction.