i think this USE flag is no longer required. On my machine kdeplasma-addons compile without errors if I removed qt3support flag. Also I checked sources (CMakeLists.txt, *.h, *.cpp) using grep and nothing found related to Q3 and QT3_SUPPORT
Currently kde4 eclass is based on hard-depending on qt3support. It might be good idea to revisit this behaviour since kde-4.7 to slowly phase qt3support out.
Ah the reason is wontfix as it is longterm goal which none will focus in short term (expect it to show up in 2012)