$\mathrm{HCN}$ and $\mathrm{HNC}$ molecules are formed by the same atoms.The correct set of formal charges for HNC is

Structure of HCN and HNC is:

Formal charge on H, C, and N in HCN is:

$$\begin{gathered} \mathrm{Formal} \mathrm{charge} \mathrm{of} \mathrm{H}=1-0-\frac{1}{2}\times 2=0 \\ \mathrm{Formal} \mathrm{charge} \mathrm{on} \mathrm{N} =5-0-\frac{1}{2}\times 8=+1 \\ \mathrm{Formal} \mathrm{charge} \mathrm{on} \mathrm{C}=4-2-\frac{1}{2}\times 6=-1 \end{gathered}$$

Therefore, the correct option is (3).