What is the correct name for SO3?

The correct name for ##SO_3## is sulfur trioxide.
Using the rules for the nomenclature for naming binary covalent (molecular) compounds the compound ##SO_3## is named sulfur trioxide. The prefix tri- indicates that the oxygen has a subscript of 3 and that a molecule of sulfur trioxide has three oxygen atoms. In this chemical formula sulfur does not have a written subscript which is understood to be 1. You might wonder why it is not called monosulfur dioxide. By convention chemists do not add the prefix mono- to the name of the first element in the binary covalent compound. However if the second element has no subscript it will be given the prefix mono-. For example the binary covalent compound CO is named carbon monoxide not carbon oxide and not monocarbon monoxide.
Comment:
I always get confused on SO3 as a molecular compound and SO3 as a polyatomic ion why are they expressed the same way? Kaneki
When the compound sulfur trioxide (##SO_3##) is indicated there won’t be a charge associated with the formula. When the sulfite ion is indicated it will have a charge of 2- associated with it (##SO_3^2-##).