Okay, click on "Page Layout", set the mode to "Single Page".
Click on "Page Setup...". In Page Setup" use the "Paper Size" drop-down to select "Manage Custom Sizes". If A3 size is already defined select it. If not then click on the + at the bottom left, name the paper size and set the width and height with no margins, then OK. Click Landscape and OK again.
In the "Page Layout" dialog box, verify that the page width and height are right. Click on "Overlay Drawing in Preview", click on "Fit To Area", Then click on "Align Center", and un-check "Preserve Annotation Sizes". You can adjust how much paper your drawing uses by changing the "Scale Ratio", a little bit goes a long way here and you may need to click on "Align Center" to re-center it. When done, click on "OK"
Select "Print". The preview should look right, now you can print or save as a pdf.
Note that when defining your paper size in the Page Setup dialog box use either "User Defined" or select a printer that supports A3. If the selected printer in this box does not support A3 then it will not allow you to set that size.