.123d is Autodesk 123D Drawing
Key Features: .123D File Extension
What is an .123D file?
CAD drawing created by 123D, a free 3D CAD design application provided by Autodesk; may contain multiple parts that together comprise the model; can be published to standard raster image formats or to Autodesk's .DWG format.
123D is often used for prototyping since it is not as robust as AutoCAD. Therefore, models are often used for showcasing ideas as mesh models, props, and scenery. Many free 123D models are provided at the 123D website for download.
NOTE: 123D was discontinued in March 2017.