SO3 - Sulfur Trioxide:
First draw the Lewis dot structure:
SO3 Lewis Structure
Electron geometry: trigonal planar. Hybridization: sp2

Then draw the 3D molecular structure using VSEPR rules:
Click and drag the molecle to rotate it.

Decision:
The molecular geometry of SO3 is trigonal planar with symmetric charge distribution on the central atom.
Therefore SO3 is nonpolar.

Sulfur Trioxide on Wikipedia.
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