When and who introduced the idea of a scene graph?
Not 3D as such, but it seems in 1963 Ivan Sutherland's seminal &quot;Sketchpad&quot; (and wikipedia) had instancing. eg. See &quot;Chapter VI RECURSIVE FUNCTIONS&quot; of the 2nd reference.
IIRC somewhere there's a video of it in action - it made me annoyed as to how some of today's vector drawing tools don't seem to be able to do some of what sketchpad appeared to support.
Update: @Joojaa has kindly located a video   or this