There are three equivalent Lewis structures for CO32.
1
1
1
2
2
2
1
2
1
2
1
4
2
For SO3, two equivalent resonance structures are sufficient to describe the bonding.
SeBr2O only has one lewis structure. you can fill all of the lone pairs without having to use a double bond (double bonds are why there are equivalent lewis structures)
2
Two equivalent Lewis structures are necessary to describe the bonding in O3. These structures involve adding a double bond between each pair of oxygen atoms in alternate arrangements.
A single Lewis structure can be used to represent the bonding in SeF2O.