Which prefix is needed when naming so3?

The formula shows that molecule is made of one sulfur atom and three oxygen atoms. This means that the prefix used should be 'tri'. This means three and the substance would be called sulfur trioxide. A molecule with two oxygen atoms would be named sulfur dioxide. The nomenclature of chemicals often uses prefixes so the name itself can give a picture of what it is made of.

They are all full and don't need any more electrons in there outer shell.