Yep. I used windowmaker then enlightenment ( I fell prey to shiny baubles ) then kde3 ( yes, in my mind still the single best desktop environment I’ve ever used ), and now i3. I love i3.
The functionality that was there at the time is astounding. Kmail was better than Evolution (minus the Exchange support), KOffice was half decent and a lot more performant than OpenOffice. KDevelop was a quite progressive IDE. Heck even Konqueror was a decent browser, with an engine that was so well designed and portable, that when Apple decided to make a browser, they forked KHTML.
Everything was cohesive, used a unified design language, worked great on a system with 1GB of RAM.
I don't understand how all these years later, on systems infinitely more powerful, desktop Linux is yet to get to a similar state. KDE4 was such a disaster, I left KDE and have never really gone back since.
They decided they would not do the same mistake of releasing such unstable versions again in the future, and indeed, plasma 6.0 was usable since day one.
Have you tried Plasma 6 or a recent plasma 5? If you liked KDE 3, you might like it. I find it's a really pleasant environment, I'm not sure something from KDE 3 is missing. It's also quite lightweight, I run it on a tablet with 1GB of RAM, it's a bit slow but quite usable. The web is pretty much unusable though for the most part unfortunately.
There's also a big work of ironing out the small bugs and they are working on making things more cohesive with a design system and some work to unify qml-based and qtwidget-based components (yes, on that front, qml was not a thing when KDE 3 was released, the UI was probably more cohesive - though Breeze, KDE's theme, already does a lot for this and is beautiful).
I don't know Trinity but that's of course another option.
CADT ?
They redesign the whole thing every couple of years instead of fixing bugs.
I forgot how it was called, but one could easily automate the whole desktop back then via scripts and an interface to the KDE3 programs.
KDE IMHO never returned to that level of polish and quality (yes, I tried 6).
Today I am a GNOME user and its killer feature for me is provisioning via dconf and Evolution is my favorite email client on any platform.