The third period contains only eight elements, even though the electron capacity of the third shell is 18. Why?

The third period contains only eight elements, even though the electron capacity of the third shell is 18 because after 8 electrons go to the third shell, the next one is added to the fourth shell. Actually, the 3rd shell should contain 18 elements, but as per Aufbau’s principle, electrons should be filled in the abilities according to their increasing order of energies. It is seen that 4s orbital has lower energy than 3d orbital i.e the electrons fill in this order 3s<3p<4s<3d. So, 8 electrons fill the 3s and 3p orbitals then fill the orbital in the 4th shell. So, of the 18 electrons, there are 10 electrons which are found to have a high energy equivalent to the energy of the 4th shell. Hence, these electrons are not counted in the 3rd shell and only 8 electrons are in the 3rd shell, resulting in the 2, 8, 8, 18, electronic configuration.