what you mean with "is possible to use kde3" - of course it would be possible to use kde in SliTaz, if you compile it, but it does not make sense - so there is no kde available for SliTaz - there is fluxbox and xfce available beside openbox.

If you want to have a lightwight KDE distro use Slax - http://www.slax.org/build.php

We have libraries to use Qt4(.6) applications (not KDE 4) but not Qt3 (so not KDE 3).

not make sense because of higher sys requirements?
i was reading here: http://forum.slitaz.org/index.php/discussion/39/power-management-how-to
that slitaz 3 require more ram than xubuntu, it's true?

try it out - take a computer with 64 or 128 MB Ram and try to boot slitaz-lowram.iso and xubuntu - you will know

No, I've tried both. Slitaz uses less ram than xubuntu when installed. You can't compare a liveCD with an installed xubuntu and vice versa. Either compare LiveCD's or installs.

Either way it kicks xubuntu's ass, mostly because xubuntu's badly configured - like most *buntu versions. Some people recommend lubuntu over xubuntu because it uses LXDE (like Slitaz) over XFCE and it's default configuration and apps are way better than xubuntu's one. For a more out of the box experience that works out of the box (flash, codecs, ala video players), most people recommend Mint which is based on Ubuntu. There is also an LXDE flavour of it, which is super light. Slitaz STILL uses less RAM than even the LXDE based Ubuntu and Mint though...

Oh and you could also probably convert over the debian kde packages and it's dependacies using the tazpkg convert command. This could technically get KDE to work with Slitaz too and wouldn't require compiling, but converting could take a while and you may have to install pretty big packages for KDE to work well...