title: WPF: How to add 3D object from xaml file to my viewport3D?
body: <p>I have viewport3D and I want to add multiple 3d objects defined in xaml files to this Viewport3D, but by default 3d object deffined in .xaml file comes already with viewport3D and I can not add it to my Viewport3D
  How I can use those 3D objects from .xaml files in my Viewport3D?</p>
  
  <p>I tryed to read xaml with XamlReader cast it to viewport3D and then take vieport3D child elements and add to my viewport, 
  but I get error "Specified Visual is already a child of another Visual or the root of a CompositionTarget."  </p>