I can export single object to json file..but most free models which I downloaded it from the internet consist itself from multi objects in the scene .. so I need to download the scene Ex : the building model consist of Door object - window object..ect
The Blender exporter might still support this, but you'd need to switch from THREE.ObjectLoader to THREE.JSONLoader to import your model into three.js. I'm not sure how well-supported that is overall.
The other option, possibly more reliable, would be to use another of Blender's export formats that three.js also supports. I'd suggest trying FBX and glTF (addon) to get started. These work with THREE.FBXLoader and THREE.GLTFLoader respectively.