Currently GTK3 support is only tested with x11/libxfce4menu, however with some new panel plugins, we need also to know if GTK3 option is set in x11-wm/xfce4-panel (by default it's on).
E.g., in audio/xfce4-pulseaudio-panel, USES framework checks if GTK3 is set for x11/libxfce4menu and x11-wm/xfce4-panel, which seems better.