Answer
$$Fe\gt Fe^{2+}\gt Fe^{3+}$$
The radii of $Fe$ is larger than those of $Fe^{2+}$ and $Fe^{3+}$ since cations are smaller than the corresponding neutral atoms.
The radii of $Fe^{2+}$ is larger than that of $Fe^{3+}$ since it has more electrons, so there is more electron-electron repulsion to screen the attraction from the nucleus.
Work Step by Step
$$Fe\gt Fe^{2+}\gt Fe^{3+}$$
Cations have fewer electrons than their corresponding neutral atoms, but still have the same nuclear charge. That means in cations, there is less electron-electron repulsion, making the electrons more attracted to the nucleus and as a result, move closer to the nucleus.
Therefore, cations are smaller than their corresponding neutral atoms.
That explains why the radii of $Fe$ is larger than the radii of $Fe^{2+}$ and $Fe^{3+}$.
$Fe$ has 26 electrons. In the case of $Fe^{2+}$, only 2 electrons are removed, leaving back 24 electrons. In the case of $Fe^{3+}$, 3 electrons are removed, leaving back 23 electrons.
24 electrons would have more electron-electron repulsions than 23 electrons. That means $Fe^{2+}$ would be less attracted to the nucleus than $Fe^{3+}$, due to its having more electron-electron repulsions to screen the attraction.
As a result, the radii of $Fe^{2+}$ would be larger than the radii of $Fe^{3+}$.
