Extended reality technologies -- mixed, augmented, and virtual reality, and future-related technologies -- are rapidly expanding in many fields, with underexplored potentials for multimodal composition in digital media environments. This research generates new knowledge about the novel wearable technology -- smart glasses -- to support elementary students' multimodal story authoring with 3D virtual objects or holograms. The researchers and teachers implemented learning experiences with upper elementary students from three classrooms to compose and illustrate written narratives before retelling the story with Microsoft HoloLens 2 smart glasses, selecting 3D holograms to illustrate the settings, characters, and events from the 3D Viewer software. The findings analyse how smart glasses supported students' multimodal composition, and relatedly, the new modal resources available to students wearing smart glasses to compose 3D stories. The findings have significance for educators and researchers to understand and utilise the multimodal affordances of augmented and mixed reality environments for composing and storytelling.