This is a free service so there are natural limits to what we can provide, unfortunately we do not include the printing (or customisation) of the material we've provided.
If you wish to print the poster in A3, the best way is to save the poster first as a PDF.
To do this simply select the Print button when you visit the poster link, select 'Print to PDF' on the print dialog box, and a PDF copy of the poster will be saved to your computer. The PDF can then be printed in a size that meets your requirements.