A bit more info would be useful, i.e. is this an architect's drawing or similar?
The easiest way to copy an A3 document is to take it to a copy shop as they will have a large enough photocopier and a stock of A3 paper.
If you want to use an ordinary photocopier such as they have in libraries, then you could fold the A3 and photocopy each side onto A4 paper then sellotape them together.
If you want a scan, and you've got the kind of printer/scanner that I have, you can feed the A3 in sideways and then depending on your software you'll either end up with a scanned A3 version or you'll have to scan each half separately and use software to "stitch" them together to make a full version.
That's for a full-sized version. If you only need an approximate version, you could take a photo of it with a digital camera and transfer that to your computer.