SO3 is a molecule with no polarity. There are three S-O bonds in the molecule and no non-bonding electron pairs. The molecule is non-polar due to its trigonal planar structure.
A mismatch in electronegativity causes polarity. The electrons are drawn to oxygen because it is more electronegative than sulphur. Because the electrons in the S-O bond are scattered unevenly, the oxygen has a negative charge and the bond has polarity.
The three S-O bonds are aligned in the same plane, each 120 degrees apart. The polarities cancel out due to the trigonal planar shape, resulting in a non-polar molecule.