To solve this problem, we are tasked with determining the maximal possible number of colours that can be present in an $n \times n \times n$ cube, considering the described constraints.

### Analysis of the Problem

1. Cube Composition:
The cube consists of $n^3$ unit cubes.

2. Box Layers:
For each orientation of the cube, there are $n \times n \times 1$ layers:
- There are 3 groups of $n$ layers corresponding to each spatial dimension (axes) $x, y, z$. Thus, the total number of layers considered is $3n$.

3. Colour Sets:
- Each layer gives rise to a set of colours present in that layer.
- The problem states that for any colour set in one group of layers, the same set appears in each of the other two groups.

4. Objective:
Our goal is to maximize the number of distinct colours that can appear in the cube under these constraints.

### Solution Approach

Let's examine how the condition of repeated sets across the three groups can be satisfied. A uniform approach can be broken down into smaller components:

1. Diagonal Slots and Leafs:
One effective way to construct such overlap of sets is to use the "diagonal" or "leaf" method for enumeration.

2. Arithmetic Progression:
Consider a logical assignment through the summation of diagonals which associate with triangular numbers. This approach simplifies the distribution of these sets based on a symmetrical scheduling.

3. Triangular Numbers:
When constructing using diagonals, the maximum distinct colours that can be utilized corresponds to the sum of the first $n$ triangular numbers:

$$\text{Sum of first } n \text{ triangular numbers } = \sum_{k=1}^{n} \frac{k(k+1)}{2}$$

4. Evaluation:
- This simplifies to computing the sum of triangular numbers formula:

$$S_n = \frac{n(n+1)(2n+1)}{6}$$

This formula computes the total number of distinct colours in such a cube configuration, where the repetitions across groups remain correct and balanced.

### Conclusion

The maximal possible number of colours, under the given configuration, can be expressed as:

$$\boxed{\frac{n(n+1)(2n+1)}{6}}$$

This result arises from the requirement that any given set must appear exactly once in each of the three box orientations, and aligns with the computed sum of triangular numbers.