You don't need to download the files that have "-18n-" in the filename, unless you intend to run in some language other than English. You also don't have to download the files that have "devel" in the filename unless you want to do some development work, but you can always go get them later. Download all the others to some temp directory (I use a temp directory under my /home/<my name> directory). Then run the following, according to the "readme" file:
cd /where/you/downloaded/the/KDE3/RPMS
rpm -e `rpm -qa |egrep ^kde`
rpm -Uvh *rpm
Hopefully, RedHat has these binaries set up so that you will get no conflicts or dependency errors. The usual wisdom is to run rpm -Uvh on the following sequence of files:
arts
kdelibs
kdebase
everything else except kdeaddons
kdeaddons
Try the RedHat instructions first, then come back with any errors that you may be getting. Good luck
!!