Some of the properties of the two species, NO3- and H3O+, are described below. Which one of them is correct?

a. In NO3-, Hybridisation = 1/2[5 + 0 + 1] = 3. Thus, in NO3-, the central atom is sp2 hybridised and it has trigonal planar geometry.
In H3O+, Hybridisation = 1/2[6 + 3 - 1] = 4. Thus, in H3O+, the central atom is sp3 hybridised and it has pyramidal geometry.
Therefore, NO3- and H3O+ are dissimilar in hybridization for the central atom with different structures.