Question

why in calcium only 8 electron are present in the third shell

Answer 1

The is an important principle called as the Aufbau principle. This principle states that, “Electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels. In this way, the electrons of an atom or ion form the most stable electron configuration possible.” 
According to this principle, Calcium has a configuration of 1s2 ,2s2, 2p6 ,3s2, 3p6, 4s2.

This is the reason for Calcium to have a configuration of 2,8,8,2 instead of 2,8,10.