The 3D skin was a part of the ASP.NET Classic RadControls but it was not included in the ASP.NET Ajax RadControls. If you are seeing this skin in the demos I'm suspecting that you might be looking at the ASP.NETClassic demos rather than the Ajax RadControls Demos (found here). If you do want to use this skin with the latest versions (although there might be some functionality not accounted for) you can see a list of the old skins here, then use the Telerik Skin Converter and convert the older skin to a file that works with the latest version of the controls.