For ${\mathrm{Ca}}_3{\mathrm{N}}_2$

${\mathrm{Ca}}^{2+}{\mathrm{N}}^{3-}$

$$\begin{gathered} 3(+2)+\mathrm{x}\left(2\right)=0 \\ ,\mathrm{x}=-3 \end{gathered}$$

For ${\mathrm{Ca}}_3{\mathrm{P}}_2$

$$\begin{gathered} 3(+1)+\mathrm{x}=0 \\ =-3 \end{gathered}$$

For ${\mathrm{Zn}}_3{\mathrm{Sb}}_2$

$$\begin{gathered} 3(+2)+\mathrm{x}\left(2\right)=0 \\ ,\mathrm{x}=-3 \end{gathered}$$

For ${\mathrm{Mg}}_3{\mathrm{Bi}}_2$

$$\begin{gathered} 3(+2)+\mathrm{x}\left(2\right)=0 \\ ,\mathrm{x}=-3 \end{gathered}$$

For ${\mathrm{Mg}}_3{\mathrm{N}}_2$

$$\begin{gathered} 3(+2)+\mathrm{x}\left(2\right)=0 \\ ,\mathrm{x}=-3 \end{gathered}$$

The non-metals which show a $-3$ oxidation state is,

${\mathrm{Ca}}_3{\mathrm{N}}_2,{ \mathrm{Ca}}_3{\mathrm{P}}_2,{ \mathrm{Zn}}_3{\mathrm{Sb}}_2,{ \mathrm{Mg}}_3{\mathrm{Bi}}_2,{ \mathrm{Mg}}_3{\mathrm{N}}_2$