The third beta of KDE 4.0 is aimed at further polishing of the KDE codebase and also marks the freeze of the KDE Development Platform. Since the last Beta, most of KDE has been frozen for new features, instead receiving the necessary polish and bugfixing. The components which were exempt from this freeze, like Plasma, saw significant improvements.
http://www.kde.org/announcements/announce-4.0-beta3.php