3DXML format (Dassault Systems Universal 3D XML file) is a proprietary format generally only available to software produced by Dassault Systems, for example CATIA. As such, your best option for a .step (STEP product data) to 3DXML conversion would be to use CATIA, or any other similar software that has licensed the use of 3DXML format.
Open STEP file Open 3DXML fileA 3D product data optimalization software