Well, that depends on your skills. If you really want to have a specific KDE version, there is nothing wrong with pointing you to one of the favourite distros.
Even on
http://www.kde.org/download/ under "KDE 3 series" there is a list of links to distros.
You could for sure download all the needed packages and compile them by yourself. But that's not the easiest step.
If you want it easy, just stay on your distro (according to another thread it is ubuntu 8.04) and use your package manager. There you find the 3.5.x desktop as an option. Simply search for kubuntu-desktop. It is based on the KDE3 series, whereas kubuntu-kde4-desktop is based on.... well, KDE4.
So try it this way, or try it the hard way by the above link.