Sulfur trioxide structure
The answer depends on which teaching you follow.

Traditional teaching, dating back to Linus Pauling, would prefer three double bonds to minimise charge separation, requiring an expanded octet.

Even older traditional teaching would favour the validity of the octet rule and take charge separation as a tolerable by-product.

Both sides have their advantage and disadvantage:


The Pauling method


correctly shows that all three bonds are equal without resorting to mesomery
incorrectly assumes a bond order of $2$ rather than the actual $1.4$

The charge-separation method


predicts a bond order of $1.33$ which is much closer to $1.4$
requires mesomery of three structures to explain the three equal bond lengths.



I for my part prefer the mesomeric charge-separation description, because we have no trouble drawing benzene in the same way and because I like the octet rule. Others will disagree and prefer the three double bonds.

I wouldn’t know anybody who prefers a description with two double bonds and one single bond. It is just between the two models making it all the less correct.

Please also see this excellent answer to a related question by Martin which includes molecular orbitals for $\ce{SO3}$