Here's why:

* Principal quantum number (n): This number describes the energy level of an electron. n = 3 means the electron is in the third energy level.

* Angular momentum quantum number (l): This number describes the shape of the subshells. It can take on values from 0 to n-1.

* For n = 3, l can be 0, 1, or 2.

* l = 0 corresponds to the s subshell (spherical)

* l = 1 corresponds to the p subshell (dumbbell-shaped)

* l = 2 corresponds to the d subshell (more complex shape)

Therefore, for n = 3, you have the 3s, 3p, and 3d subshells.