In broad sense, 3d to json conversion describes export of '3D' data to json format, which is not really common, but not really impossible. This is often done for export of meshes for very specific purpose and not for data exchange.
The most mentioned course of action is to do that with Blender, as it allows export of meshes to json. You just have to make sure your '3d' format is supported in Blender, otherwise you have to convert it first to one of these formats.