In each structure, the sulfur atom is bonded to the three oxygen atoms with double bonds, and the oxygen atoms have one lone pair of electrons each. The three structures differ only in the arrangement of the double bonds and lone pairs around the sulfur atom.
The three equivalent Lewis structures of SO3 are:
```
O=S=O
|
O
O=S-O
||
O
O-S=O
||
O
```
