If the error is repeatable, then it's either a bad disc or a dirty/scratched disc. Since this is an official disc, the chances of it being bad are remote. So make sure the disc is clean before do anything more involved.
If the error occurs with other discs or with other packages on the disc, it may be a problem with your CD-ROM drive. But that probably shouldn't occur only on Disc 3.
You can download ISO's of RHEL from the Red Hat Network if you have a valid service account there.