Are there particular passages, ouvertures, moments, whole operas... from these three composers that you think have particularly effective, striking and beautiful orchestration?
Perhaps a random note: Riccardo Chailly talked in one interview about Verdi's masterpieces of orchestration and mentioned as an example the dark, thick, black, funèbre sound of the last act of Aïda. He also mentioned Mahler's fascination with the composer and that while orchestrating his Fourth he was conducting Falstaff and that the two share many colouring ideas. :)