Yes, you need to save the CorelDRAW X3 files to CorelDRAW 12 format. For this purpose, open the file in CorelDRAW X3 and select the Save As option from the File menu. After that, from the Version drop-down list, select version 12 or an older one:

If you no longer have CorelDRAW X3 on your computer, download a trial version from the official web page and do the same.