If you have specified A3 paper in your document, the printer should automatically choose the correct paper.

All library printers, unless specified on the machine, should contain A3 paper.

To create an A3 document in Microsoft Word, start a new document and in the Page Layout tab click Size, then choose A3 from the drop-down list. You are now working on an A3 size document, and it should automatically print accordingly. ​

If you have an existing Microsoft Word document that you would like to scale up to A3 size: 

Open the document and click File > Print.  Click Printer Properties then choose the Effects tab. Select Print document on: and set it to A3. Ensure Scale to fit is ticked. Click OK. 

Click the large Print Icon to send the job to your queue. When you print the document it will automatically print on A3 paper and stretch the content to fit the larger page.