I don't know about the Amiga, but I recently researched an A3 photo printer because I was thinking of making A3 transparency prints for Holby City (there was a printer setup problem at work to do with the X-ray films).
Anyway I checked things out, making my current Canon i950 the baseline and not wanting to drop below that in terms of quality. The winner (in my opinion) for A3 photo prints is the Epson R1800. Pop along to Photo-i and read the
review.But to use that on the Amiga, well good luck.