There are seven resonance structures for ##”SO”_3##.

When you draw the Lewis structure, you first get the three structures at the top.

In each of them, ##”S”## has a formal charge of +2 and two of the ##”O”## atoms have formal charges of -1.

In each of the three structures in the middle, ##”S”## has a formal charge of +1 and one of the ##”O”## atoms has a formal charge of -1.

In the bottom structure, all atoms have a formal charge of zero.

From the point of view of formal charges, the bottom structure is the most stable structure.