Hi -- Do you have your 3D data published to the web yet? If not, you should start over in the 3D Cities / Campuses or 3D Mapping places to learn how to create models and publish them to the web using ArcGIS Pro, CityEngine, and ArcGIS Online. Once you have your 3D scenes created then you can add them to your story map as described in the blog article I linked to above.
Owen Evans
Lead Product Engineer | StoryMaps