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Create your numbered PDF file with three copies of each page (ie 1,1,1,2,2,2,3,3,3,4,4,4,5,5,5,...). - Use the repeat function in Numbering Star
Then from your Fiery use Mixed Media > New Page Range to set up Page 1 from tray 1, Page 2 from tray 2 and Page 3 from tray 3 (or whichever...