in general group 2 of the periodic table of elements has the highest 3rd ionization energy. This is because a doubly ionized element from group 2 will have the same electron configuration as an inert gas, which is very stable. Removing one more electron means making it less stable as for electronic configuration is concerned. Also, to remove an electron from a doubly ionized atom where the electron is closer to the nucleus is more difficult than removing it from a singly ionized one, where the electron is further away from the nucleus.
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