Mathematically, the extension to 3D is trivial. Computationally, the difference is enormous. You're moving from O(n²) cells required to fill a space to O(n³).

Mathematically, the extension to 3D is trivial. Computationally, the difference is enormous. You're moving from O(n²) cells required to fill a space to O(n³). Rendering also becomes significantly more complex, as you wouldn't be able to see anything interesting if it remained fully opaque.