#H_2O# has 4 electron electron pairs, 2 bonding and 2 non-bonding, that assume the shape of a tetrahedron, Two of the lobes are non-bonding (i.e. lone pairs), so we describe the molecular structure as bent. #H_3O^+# also has 4 electron pairs, 3 bonding, and 1 non-bonding, that also assume the shape of the tetrahedron. But we describe molecular geometry on the basis of bonding pairs. So the hydronium ion is described as pyramidal; precisely the same as in ammonia, #:NH_3#, a neutral molecule.