The three people can be represented by letters S, M, T for small, medium, tall.
These letters can be arranged in 6 different ways one of which is TMS. So the probability is 1/6.
Merely comparing their heights in line we still have 6 possible outcomes:
For 3 people A, B and C:
- A>B<C, and A>C
- A<B>C, and A>C
- A>B<C, and A<C
- A<B>C, and A<C