$\quad \quad \quad \quad _{25} \mathrm{Mn} \quad \quad \quad _{26} \mathrm{Fe} \quad \quad \quad _{27} \mathrm{Co} \quad \quad \quad \quad _{28}Ni$

$\mathrm{M}= \quad [\mathrm{Ar}] 3 \mathrm{d}^{3} 4 \mathrm{s}^{2} \quad[\mathrm{Ar}] 3 \mathrm{d}^{6} 4 \mathrm{s}^{2} \quad[\mathrm{Ar}] 3 \mathrm{d}^{7} 4 \mathrm{s}^{2} \quad[\mathrm{Ar}] 3 \mathrm{d}^{2} 4 \mathrm{s}^{2}$

$\mathrm{M}^{2+}=[\mathrm{Ar}] 3 \mathrm{d}^{3} 4 \mathrm{s}^{0} \quad[\mathrm{Ar}] 3 \mathrm{d}^{6} 4 \mathrm{s}^{0} \quad[\mathrm{Ar}] 3 \mathrm{d}^{7} 4 \mathrm{s}^{0} \quad[\mathrm{Ar}] 3 \mathrm{d}^{2} 4 \mathrm{s}^{6}$

So third ionisation energy is minimum for $Fe$