You're correct that the third shell (or energy level) has a maximum capacity of 18 electrons. However, when we examine the electron configuration of argon (Ar), we find that it has a total of 18 electrons distributed across multiple energy levels.
The electron configuration of argon is 1s² 2s² 2p⁶ 3s² 3p⁶, indicating that it has a full set of electrons in its third energy level. The first two energy levels, 1s and 2s, are completely filled with 2 electrons each, while the 2p sublevel has 6 electrons. Therefore, when we add the 2s and 2p electrons together, we get a total of 8 electrons in the second energy level.
In terms of the third energy level, the 3s sublevel is completely filled with 2 electrons, and the 3p sublevel is also completely filled with 6 electrons. Consequently, argon has a full complement of 8 electrons in its third energy level. The remaining 2 electrons (to reach its atomic number of 18) occupy the 1s sublevel in the first energy level.
Although the third energy level can hold more electrons, the electron configuration of argon results in a completely filled second and third energy level, giving it a stable and inert electronic structure.