User blog comment:ArtismScrub/3D Sudoku idea/@comment-35470197-20190307011811

For the playability, human beings are usualy unable to recognise and memorise the configuration of 9 * 9 * 9 numbers. Therefore it might be better to consider the alternative 3D game for 4 * 4 * 4 first. Namely, look at all 4 * 1 * 1-type lines and 2 * 2 * 1-type sequares.

If you prefer to refer to the sum at cubes, then it is better to consider 8 * 8 * 8. Then look at all 8 * 1 * 1-type lines, 4 * 2 * 1-type squares, and 2 * 2 * 2-type cubes. It is much easirer than your original game, not only because 8 * 8 * 8 is much smaller than 9 * 9 * 9, but also because the game has more restrictions so that players need to consider less patterns.

As another approach, let consider the completed cube, i.e. an answer of a problem in your original 3D game. Then delete only 27 distinct digits from the cube. Filling the 27 branks is sufficiently playable as long as you have a way to display the cube appropriately.