So they all play the same, and they sound the same to you when you play them. Maybe you could grab a friend and have him or her play all three while you listened...blindfolded. Who knows - they might sound different from the other side.

Maybe if you keep visiting them and hold off on the decision, one will begin to stand out in your mind.

If that doesn't work, I'm with Spaw. Pick the one with the coolest case.