While going through my book, I came across a problem which gave some options about the order of energy of 3s, 3p and 3d orbitals of hydrogen atom. The correct answer was given as:
3s, 3p and 3d orbitals all have the same energy
I also read somewhere that the order of energy of different orbitals increases as the value of $n+l$ increases where $n$ is the principal quantum number and $l$ is the azimuthal quantum number. So my guess was $3s<3p<3d$ .
What is the reason for this?