The ability of head-up displays (HUDs) to present information within the usual viewpoint of the user has led to a quick adoption in domains where attention is crucial, such as in the car. As HUDs employ 3D technology, further opportunities emerge: information can be structured and positioned in 3D space thus allowing important information to be perceived more easily and information can be registered with objects in the visual scene to communicate a relationship. This allows novel user interfaces to be built. As of today, however, no prototyping tools exist, that allow 3D UIs for HUDs to be sketched and tested prior to development. To close this gap, we report on the design and development of the 3D Head-Up Display Designer (3D-HUDD). In addition, we present an evaluation of the tool with 24 participants, comparing different input modalities and depth management modes.