Nitrogen exhibits the oxidation number $-\frac{1}{3}$  in the compound

In ${\text{N}}_{\text{3}}\text{H}$  molecule, hydrogen has $+\text{1}$  oxidation state and nitrogen shows $-1$  oxidation state. Since electro negativity of nitrogen is higher than the electro negativity of hydrogen. In ${\text{N}}_{\text{3}}\text{H}$ , three N atoms sharing a single uni negative charge $-1$ at ${\text{N}}_3{}^{-}$ ion. Therefore, the oxidation state of each “N” atom must be equal to $-\frac{1}{3}$ .