The ionic radii of ${\mathrm{N}}^{3-},{\mathrm{O}}^{2-}\mathrm{and}{\mathrm{F}}^{-}$ are respectively given by

$\begin{array}{l}\mathrm{Radius}\propto \frac{1}{\left(\mathrm{Z}/\mathrm{e}\right)}\\ \mathrm{Order}\mathrm{of}\mathrm{radius}={\mathrm{N}}^{-3}>{\mathrm{O}}^{-2}>{\mathrm{F}}^{-}\end{array}$

 another explanation: In case of isoelectronic species , size increases with negative charge 

                                      and size decreases with positive charge