Here's why:

* s orbital: The third energy level starts with an s orbital, which holds a maximum of 2 electrons.

* p orbitals: It also includes three p orbitals, each holding a maximum of 2 electrons, totaling 6 electrons.

* d orbitals: Finally, it has five d orbitals, each holding a maximum of 2 electrons, for a total of 10 electrons.

Therefore, the total number of electrons the third energy level can hold is 2 + 6 + 10 = 18 electrons.