- 3s: This represents the s subshell of the M shell. It has one orbital and can hold a maximum of two electrons.

- 3p: This represents the p subshell of the M shell. It has three orbitals (px, py, and pz) and can hold a maximum of six electrons.

- 3d: This represents the d subshell of the M shell. It has five orbitals (dxy, dyz, dzx, dx2-y2, and dz2) and can hold a maximum of ten electrons.

The subshells are filled in order of increasing energy, so the 3s subshell is filled before the 3p subshell, and the 3p subshell is filled before the 3d subshell.