Transparencies are a special media type that have very strict requirements in terms of fuser temperature and bound draw. The simple explanation is that even Xerox transparencies can only work to a limited extent with this model; It only supports Letter and A4-sizes for that media type. Other models of printers may have a wider range of fuser temperature options, or specialized features to support larger sizes than A4. Either way, we do not recommend using A3-sized transparences with the Phaser 7800.
For information regarding what media is supported or not, you can print the Paper Tips page from the printer's Print Reference Materials menu - or - you can also visit the Paper and Paper Tray Specifications page for more information.