You can also solve this by writing down a comparison tree on your own. For instance, there are only a few possible orderings of 3 elements: A < B < C, A < C < B, B < A < C, B < C < A, C < A < B, and C < B < A. If you compare C to A and find C < A, can the ordering be either the first, second, or third possibility? At this point, you might compare B and A, and come up with 1 or 2 possibilities.
You can do a similar thing for four elements  I believe it is possible to order four elements using only five comparisons.
