Unfortunately 3DXML format is not that well supported because it is proprietary and licensed, which means it's not available in many open-source 3D modeling solutions. However some, mainly commercial, CAD converters or software support it and allow users convert these CAD files to other formats like the ones from SolidWorks.
Open 3DXML file Open SLDASM fileA CAD visualizaton, analyzation and translating system