Project source files for the tutorial video: https://www.youtube.com/watch?v=JUR1qQ79eJY
This includes the modification shown in the last seconds of the video.
Here are the project files used for the tutorial, as well as some experiments I made at the time.
The tutorial scene is at:
tutorials\1 builtin drag drop\drag_drop_world.tscn
The tutorial scene in the "Drag and Drop System - Extra - Drag 3D items too.zip" has modifications made to drag the 3D item too.
It uses the can_drop_data to create the item when a dragged item is moved over the 3D area, and then moves using its process function. There is a bit of code here and there to make sure the item is created/destroyed whenever the dragged item goes over the UI and the 3D zones, but nothing too complicated.
I made this after I had already recorded the video and the voice so it didn't made into the tutorial, but I thought it was really interesting to at least show it.