Here's why:

* Principal Quantum Number (n): The number "3" in 3p and 3d refers to the principal quantum number, which dictates the energy level of the electron.

* Angular Momentum Quantum Number (l): The letter "p" and "d" refer to the shape of the orbital.

* p orbitals have l = 1, and there are three p orbitals (px, py, pz)

* d orbitals have l = 2, and there are five d orbitals.

Filling Order:

The filling order of atomic orbitals follows the Aufbau principle and Hund's rule:

1. Lower energy levels fill first. This is why 3p comes before 3d.

2. Orbitals within a subshell fill individually before doubling up. This is why all three 3p orbitals are filled before any 3d orbitals are occupied.

Therefore, after the 3p orbitals are filled, the next orbital to be filled is the 3d orbital.