Also, I vote for Indus/Harappan and Shang China to be two of the three, on the basis that they are probably the least-known to your students (unless they're from India or China), so it will be the most educational. And possibly Egypt as the third, on the basis that it'll be the one they think they know best, so they'll have the mind-broadening experience of learning that much of what they thought they knew was simplistic/wrong/etc.