Probably the easiest way is to create a new blank document in A3 size, then
copy everything except the last paragraph mark from the A4 document and paste
it into the A3 document. You'll also have to copy/paste any headers or
footers separately.
Before you start, make sure you select a printer that's able to print A3. If
the printer driver says it's limited to A4 or smaller, then Word won't even
show A3 in the page size list.