Nine Dishes on the Menu
Published in Brain Teasers
They could have ordered ABCDD their first night (finding out what D is), AEFGG the second night (finding out what G is and what A is, since they had ordered it the previous night too), and BEHII the third night (finding out what I, B and E are). This leaves C, F and H out, and since they had never ordered these dishes twice, and each came out on a different night, they should know what they are.
Today's brain teaser courtesy of Braingle.com.