Hi KateD13, OK A3 is fine, but what other specifications do you require, which type of board do you intend to use, Matte or Semi gloss, what about ink, OEM, CISS, Compatible or OEM Cartridges 3rd party ink, dye or pigment, quantity, the more information you give the better the answers..
So it will be Matt board. To start with printing full colour about 200 a month and then potentially scaling up to higher volumes. Ink wise, I don't know, I'm not sure what would be best. I'm not even sure what category this kind of printer falls into so I'm having trouble researching about it and therefore figuring out what I need!
The better and bigger you printer is the more reliable it will be (Heavy Duty cycle), plus the bigger the cartridge the cheaper it will be to print, and lastly is there a need for longevity or water proof text, then it would have to be pigment ink.
There are many A3 pigment printer on the market but I reckon you limited to two, Epson or Canon because HP wont be able to accept heavy stock.