A file with .3dd extension is a GIS data file created with ESRI ArcGlobe application software. It stores information to paths where the 3D data is located in the form of geodatabase. When a document is opened using the .3dd file, it will appear in the same shape as it was saved. ArcGlobe is a 3D visualization application that allows you to view large amounts of GIS data on a globe surface. ArcGlobe is one of the several applications that come binded together with ArcGIS software package.
3DD File Format - More Information
3DD files are stored to disc in binary file format and the internal file format of these files is not available publicly.