Assuming that you have an active maintenance contract, you can download and install Creo 3. You'll find it here (you'll need your company's maintenance info to access it):
https://support.ptc.com/appserver/cs/software_update/swupdate.jsp
If you do not have an active maintenance contract, you will have to renew your maintenance with your VAR, as Martin said.
If you simply want to open the Creo 3 files in Creo 2, you can check out the GCRI plugin info here:
http://support.ptc.com/products/granite/gcri/free-download.htm
GCRI gives you a limited ability to work with newer version files. Opening is no problem as is, I think, adding features. Editing the existing features is not possible, though.
