Comment 1 for bug 932880

You can choose from many predefined paper sizes in 'Document properties' (for the current document).

> I presume this is a bug (…)

It's certainly not a bug if Inkscape doesn't ship with a template for A3 files (at the most it's a missing "feature"), and if you frequently need such a template, you can create it anytime yourself:
<http://libregraphicsworld.org/blog/entry/using-templates-in-inkscape>