In a hydrogen atom, the 3s, 3p and 3d orbitals have the same energy…

$n = 1, 2, 3, \ldots$; $l = 0$ to $n - 1$ ($s, p, d, f$); $m_l = -l$ to $+l$; $m_s = \pm\tfrac{1}{2}$. A subshell has $2l + 1$ orbitals and a shell $n^2$. Nodes: radial $n - l - 1$, angular $l$, total $n - 1$. With many electrons, orbitals fill in order of $(n + l)$, the smaller $n$ first for a tie.