You've asked an impossibly difficult question! There are a couple basic pieces of advice I can offer, but after that, things get tricky.
- Play pieces you enjoy playing. In order to be thoroughly prepared, you'll have to practice a lot - and that shouldn't be a problem if you are playing something you like!
- Play pieces that are within your abilities. A jury is much more impressed by controlled and musical playing than they are by messy playing of slightly harder music.
Okay. Now the tricky part. Do you play pieces that represent you or pieces that you're pretty sure the jury wants to hear?? In an ideal world, I would lay my heart right out on the table for the jury to pick apart ... but my honest choice of pieces would probably lead to a rejection letter! For example, if I had an audition tomorrow and was required to play three contrasting works, I would play a composition of my own, Earl Wild's transcription of Rachmaninoff's "On the Death of a Linnet," and my own transcription of Poulenc's "Nous avons fait la nuit" (I'd have to stay up all night long making the arrangement first!). I think that would be the perfect way for me to represent myself at this exact moment; it would certainly be a fast-path to the essence of "Greg."
Unfortunately, juries don't really work the same way I think audiences do! Most audition panels would be offended by my seemingly slight program - the short pieces, my own compositions(!), two (count them: one, two) transcriptions, the sentimentality, the avoidance of anything pre-1900, my failure to showcase the extent of my technique, etc. Instead, I can tell you right now that the audition panels are going to want to hear you play a Beethoven Sonata and a substantial work by either Chopin, Schumann, or Brahms (or possibly Liszt or Mendelssohn). Juries seem to find that through these two categories, they can most easily asses a student's proficiency and musicianship. In the past, I have followed that formula to the best of my abilities and it has always worked for me. In your case, the third piece is flexible; it is less important, but it says more about who you are as a person. (It's usually safer to highlight your sophisticated side.) To contrast the Beethoven sonata and the Romantic work, I would recommend anything by Bach, Debussy, Ravel, Prokofiev, or Bartok, or a sophisticated work by a well-known modern composer.
(There are, of course, exceptions to the above; however, I believe it to be the most straightforward a safe way to go about building an audition program.)
A jury forms many assumptions about a student just by looking at his or her program. Try to find three pieces (preferably within the confines mentioned) that represent you, compliment and contrast one another, and create a striking and memorable impression. Then: play well!