On jsTree's documentation, it points out a configuration option for the drag and drop plugin called $.jstree.defaults.dnd.is_draggable:
A function invoked each time a node is about to be dragged, invoked in the tree's scope and receives the nodes about to be dragged as an argument (array) and the event that started the drag - return false to prevent dragging
You should be able to write a function which will check if it is an n3-x node.
I unfortunately don't see anything for specifying where it can be dropped within the documentation, but it looks like elsewhere on StackOverflow someone has solved this: dnd, how to restrict dropping to certain node types?