You should mount your HDD with write access before making changes.
Assume your original FC3 is installed in /dev/hda9, then you should boot a LiveCD and then:
1. mkdir /mnt/hda9
2. mount /dev/hda9 /mnt/hda9 -o rw
3. make you changes to /mnt/hda9/etc/fstab with your favorite text editor
4. save changes and reboot