This paper introduces a new type of interface '3D Head Pointer' for operation of a wearable robot arm in 3D space. The developed system is aimed at assisting the user in execution of daily chores (e.g. opening an umbrella, stirring a pot) while concurrently operating wearable robot arm. Previous researches have demonstrated the difficulty in simultaneously controlling a robot arm while uninterruptedly performing another task with one's own hands. The proposed method uses a combination of the orientation of the face and head bobbing (a forward and backward movement of the head) to manipulate spatial localization in a polar coordinate system. In a Virtual Reality (VR) environment, the proposed system and its efficiency at adapting its operating magnification (IDF) by relying on the incremental changes in head bobbing were evaluated through measurement of the instructional accuracy and time demand. Experiments were conducted with 12 participants. Results displayed an accuracy of 1.3 cm with a 4 seconds time span necessary to communicate an instruction. These results, along with the confirmed ability of an individual to multi-task while operating, suggested the effectiveness of the operation method and IDF.