how to create unity3d file using Unity 5.6.2f1personnal

the model format is fbx and i used a prefab so i need just to create a unity3d file

.unity3d files were used for the Unity3D webplayer, but that’s been deprecated for some years now and is not supported by any major browser so I’m not sure why you’d want to create that file format?

Your only solution is to download a version of Unity that can still target the webplayer and set that as a build target for your project.