Is there any Linux distribution that still comes with support of KDE3?
Also there is trinity desktop, that is based on kde 3.5. You can install it on Debian Lenny, Debian Squeeze, Ubuntu Karmic to Oneiric, RHEL 5-6, Fedora 15 and Slackware 12.2-13.1.
As of openSUSE 12.1 KDE 3 desktop is included as officially supported part of the distribution. Additional KDE 3 software can be installed from community-supported KDE:KDE3 repository.
(KDE3, not Trinity)
KDE3 is shipped with openSUSE. LiveDVD: http://susestudio.com/download/2dd3fc0c6ad40f78c4a53b0e4fdd7a8b/OpenSUSE_12.1_KDE3.x86_64-1.0.7.iso
Other distributions that include KDE3 are AltLinux (see, e.g., the kdebase package), aLinux and Pardus Corporate.
It looks like CentOS 5.7 still uses kde 3.5
Gentoo has a portage overlay for KDE-3.5: http://www.gentoo.org/proj/en/desktop/kde/