The correct order of increasing covalent character of $\mathrm{MgF}{}_2$, $\mathrm{MgO}$ and $\mathrm{Mg}{}_3\mathrm{N}{}_2$ is

The cation is common, so consider the anions $\mathrm{F}{}^{-}$ ($133\mathrm{pm}$), $\mathrm{O}{}^{2-}$ ($140\mathrm{pm}$) and $\mathrm{N}{}^{3-}$ ($171\mathrm{pm}$). Both anion charge and size increase in this order, so polarisability and covalent character increase from $\mathrm{MgF}{}_2$ to $\mathrm{MgO}$ to $\mathrm{Mg}{}_3\mathrm{N}{}_2$.