The transition metal having highest 3^rd ionisation enthalpy is :

3rd Ionisation energy : [NCERT Data]
V : 2833 KJ/mol
Cr : 2990 KJ/mol
Mn : 3260 KJ/mol
Fe : 2962 KJ/mol
alternative
$\begin{array}{rl} & \mathrm{Mn}:3{\mathrm{d}}^{5}4{\mathrm{s}}^2\\ & \mathrm{Fe}:3{\mathrm{d}}^{6}4{\mathrm{s}}^2\\ & \mathrm{Cr}:3{\mathrm{d}}^{5}4{\mathrm{s}}^1\\ & \mathrm{V}:3{\mathrm{d}}^{3}4{\mathrm{s}}^2\end{array}$
So Mn has the highest 3rd IE among all the given elements due to the half-filled(d⁵)configuration.