Four successive members of the first two transition elements are...

$\mathrm{V}^{2+}(21)[\mathrm{Ar}]^{18} 3 d^3 4 s^0$ $\mathrm{Cr}^{2+}$ (22) $[\mathrm{Ar}]^{18} 3 d^4 4 s^0$ $\mathrm{Mn}^{2+}$ (23) $[\mathrm{Ar}]^{18} 3 d^5 4 s^0$ $\mathrm{Fe}^{2+}(24) \quad[\mathrm{Ar}]^{18} 3 d^5 4 s^1$
The value of the third ionization enthalpy will be the largest in the case of manganese as the electrons in the orbitals are in half-filled stable electronic configuration and a very large amount of energy is required to overcome this effect.