Simon,

The amount of numbers contained in each grouping is something of an irrelevance. The wheel might as well only have ten numbers, 0-9. Considering the number of combinations/permutations, or how those groups can appear in 10 spins, the answer you are looking for is 3 to repeat. However, 10 spins is a relatively small number, meaning 3 to repeat is an unreliable expectation. All 10 groups/finals could easily appear in some order in the 10 spins. I really don't think the law of the third can be applied with reliability to such a small sample. As the number of spins in the interval diminishes, so does the law. It doesn't appear as strongly anywhere as it would over 37 spins.