Answer
$3\Big(\frac{h^2}{8mL^2}\Big)$
Work Step by Step
The quantized energies for an electron trapped in a three-dimensional infinite potential well that forms a e rectangular box are
$E_{nx,ny,nz}=\frac{h^2}{8m}\Big(\frac{n_x^2}{L_x^2}+\frac{n_y^2}{L_y^2}+\frac{n_z^2}{L_z^2}\Big),$
where $m$ is the electron mass and $n_x$ is a quantum number for well width $L_x$, $n_y$ is a quantum number for well width $L_y$ and $n_z$ is a quantum number for well width $L_z$.
In this problem, the electron is contained in a cubical box of widths $L_x=L_y=L_z=L$.
$\therefore\;\;E_{nx,ny,nz}=\frac{h^2}{8mL^2}\Big(n_x^2+n_y^2+n_z^2\Big)$
Therefore, the ground state is the $(1,1,1)$ state, with an energy of
$E_{1,1,1}=\frac{h^2}{8mL^2}\Big(1^2+1^2+1^2\Big)$
or, $E_{1,1,1}=3\Big(\frac{h^2}{8mL^2}\Big)$
