When is the n+l rule followed when working with energy of different orbitals?
3s, 3p and 3d orbitals all have the same energy in hydrogen atoms ( or monoelectron atomic ions ) according to the nonrelativistic Schroedinger model.
Fine measurements and relativistic models of hydrogen atoms shows there is slight difference in their energies, related to the fine structure of spectra  and Lamb shift ( Wikipedia  Hyperphysics )
For multielectron atoms, s, p, d orbitals have different energies, as there come in the play 2 major effects, which affect electron energies:

Mutual electron repulsion
Screening/shielding of the nucleus charge by other electrons.