I didn't do it with KDE3/Trinity but with KDE4/Trinity I can run a regular KDE4 session, and a Trinity session as another user either in another X server or in Xnest; but I can't run both KDE3 and KDE4 as the same user on different X servers. There is no $PATH conflict since Trinity automatically adds $PREFIX/bin in top of the $PATH. But with KDE3/Trinity ld.so may get confused if Tim didn't make the necessary internal version number upgrades (and it seems libkdecore is still at 4.2.0 so it's a bad sign… with KDE 4.5 libkdecore is at version 5.5.0).
Well, I can hear the peanut gallery in the background: the only way to know for sure is jump into the water and let everybody else know. I'm just trying to gather sufficient information to avoid anything nasty.
If either desktop freezes or crashes trying to run both concurrently, that is no big deal and probably nothing more than deleting cache files before restarting. Worst case is a library conflict that causes data corruption, but during those test I won't run critical apps like KMail.
As I said, although still inconvenient, fully exiting either desktop is better than rebooting. Not much, but still better.
Darrell