Recently, location-based services for Smart Cities became capable to proactively notify mobile users about hyperlocal information or location-based reminders once users enter clearly-specified geographical regions, better known as geofences. Up to now, geofences are mostly designed by humans through the interactive design of circular or polygonal shapes on 2-dimensional digital maps. In future Smart City applications, geofences will not only represent flat regions but also arbitrary 3-dimensional spaces such as no-fly zones for drones. This work investigates the applicability of using head-mounted displays together with touch controllers to interactively design 3-dimensional geofences within a virtual 3D representation of the world. It presents and discusses a proof of concept that enables humans to examine, create, modify, move, scale, and rotate 3-dimensional geofences and to edit its meta information within a generated 3D virtual environment of the inner city of Berlin.