The most effective way would be to .... compile it. Then the kde3 stuff is in /usr/local or /opt or wherever you choose to put in but not in /usr.
I am running Sid-installed kde4 and compiled kde3.5 with out a hitch. The appropriate startkde scripts set env variables and /etc/profile sets PATHs appropriately. -- To UNSUBSCRIBE, email to debian-kde-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org