What are the 3 Lewis structures for #NO_3^-#?
See this old answer.
In the nitrate ion, 3 of the 4 constituent atoms have formal charge; the third oxygen, formally doubly bound in the Lewis representation, is neutral. Of course the neutral charge can move to the other oxygen atoms by resonance (or rather the negative charge on the other oxygens can move to the neutral atom).
The important point to note is that in the anion, ALL the oxygen atoms are equivalent, with equivalent
We would represent the parent nitric acid in the same manner with formal charge separation:
But here of course the nitric acid is a neutral species, which the Lewis structure reflects.