An atom has 3 electrons in M shell. What is the atomic number of the element ?

Atomic number is equal to number of electrons.

M shell is the third energy level, K and L shell are completely filled.

The electronic configuration
K shell - 2 electrons
L shell - 8 electrons
M shell - 3 electrons

Total no. of electron = 2 + 8 + 3 = 13

Hence, total number of electron is 13. So, the atomic number of the element is 13.