Question

According to electronic configuration 3rd shell can accommodate 18 electrons but according to octet rule it completes 8 electrons

Answer 1

Octet rule is not widely accepted new theories are there to prove this.

hope it helps

Answer 2

Yes a third shell can only accommodate 8 electrons as This is because it is the maximum capacity of the 3rd shell and it doesn't tell about the order in which the electrons are filled.

The 2,8,18,… configuration is taught till class 10th in schools.

The answer to your question lies in knowing the real configuration of elements.

For example, configuration of O(8) will be 1s22s21s22s22p42p4 (2,6)

And configuration of Mn(25) will be 1s22s22p63s23p64s23d51s22s22p63s23p64s23d5 (2,8,13,2)

Clearly, the 3d orbital starts filling up only after the 4s is completely filled. Thus 2 electrons are filled in the 4th shell before the last 10 electrons are filled in the 3rd shell..