The correct decreasing order of radii of $\mathrm{Fe}$, $\mathrm{Fe}{}^{2+}$ and $\mathrm{Fe}{}^{3+}$ is:

Removal of electrons raises the ratio of nuclear charge to electrons, so each successive cation is smaller than the one before. The order is $\mathrm{Fe}>\mathrm{Fe}{}^{2+}>\mathrm{Fe}{}^{3+}$.