DescriptionAufbau Principle 3D (20 States).svg	A diagram of the state space of a 3D quantum well, with the first 20 states, by energy, shown (every state closer than 4.2 units to the origin)
Source	self-made,  This W3C-unspecified vector image was created with Inkscape .
Author	Inductiveload
Permission (Reusing this file)	Public domainPublic domainfalsefalse I, the copyright holder of this work, release this work into the public domain. This applies worldwide. In some countries this may not be legally possible; if so: I grant anyone the right to use this work for any purpose, without any conditions, unless such conditions are required by law.

This code reuqire you to load the module:

<< Graphics`Polyhedra`

a = Table[{x, y, z}, {x, 1, 5}, {y, 1, 5}, {z, 1, 5}];
a = Flatten[a, 2];
a = Select[
      a,
      Sqrt[#[[1]]^2 + #[[2]]^2 + #[[3]]^2] < 4.2 &];

Show[
  Table[
    Polyhedron[Cube, a[[n]], 0.3],
    {n, 0, Length[a]}],
  ViewPoint -> {3.5, 3, 1},
  Shading -> False,
  PlotRange -> {{0, 5}, {0, 5}, {0, 5}},
  Axes -> True
  ]