I imagine it's either the theme of Dark Sun Gwyndolin, which is also used in the Moonlight Butterfly fight, or Nameless Song, that plays during the credits of the first game.
If you're specifically looking for a song from III, then Aldrich's theme also has a similar sound, although the instruments are far louder relative to the voice in that song.
Those are the ones that I can think of off the top of my head.
