SO3 is a non-polar molecule. The molecule has three S-O bonds and no non-bonding pairs of electrons. The geometry is trigonal planar, resulting in a non-polar molecule.
Polarity arises due to a difference in electronegativity. The oxygen is more electronegative than the sulfur and attracts the electrons more strongly. The electrons of the S-O bond are distributed unevenly, giving the oxygen a negative charge and giving the bond a polarity.
The three S-O bonds lie in the same plane each 120 degrees away from the others. The trigonal planar geometry results in the cancellation of the polarities, and a non-polar molecule is formed.