I'm used to design flyers and i work in 600dpi (flyers are not A3). The most important for this kind of projetcs is 1/ CMJN 2/ a marging for the printer 3/ use a good color profile for u're monitor as to be able to control what will be printed in real time: the result is never what u see on u're screen! 4/ Be sure of the format (EPS, TIF, PSD etc...) needed I would advise u to work in 400dpi to be secure with the printer.