If you're comfortable with the command line, you can create a disk image of the directories you want backed up, then use a command-line program called hdiutil to split the resulting disk image into 3 parts before burning each image to a DVD. You'll need to have free space on your hard disk of twice the size of the files you're backing up with this method - if you're backing up 12 GB of data, you'll need 24 GB of scratch space.
Otherwise, I don't have any other ideas.