The transition metal having highest 3rd  ionisation enthalpy is :

The transition metal with the highest 3rd ionisation enthalpy is Chromium (Cr).

Here's why:

1. Ionisation enthalpy is the energy required to remove an electron from a gaseous atom or ion. The 3rd ionisation enthalpy refers to the energy needed to remove the third electron.

2. Transition metals are the elements found in Groups 3-12 of the periodic table and they include elements like iron, copper, nickel, silver, etc.

3. Among these, Chromium has the highest 3rd ionisation enthalpy. This is because after losing two electrons, the Chromium ion achieves a stable half-filled d-orbital configuration (3d5 4s1). Removing another electron would disrupt this stable configuration, thus requiring a lot of energy.

4. Therefore, Chromium has the highest 3rd ionisation enthalpy among the transition metals.