Arrange nine red cubes, nine blue cubes and nine yellow cubes into a large $3$ by $3$ cube.
No row or column of cubes can contain two cubes of the same colour.
In the picture, the top face and the right face have one of each colour.
However, the third face has two columns which contain two cubes of the same colour, so this is not correct.
You may like to try Nine Colours
once you've had a go at this task.
Printable NRICH Roadshow