Try this
extends Tree
# class member variables go here, for example:
# var a = 2
# var b = "textvar"
func _ready():
var root = self.create_item()
root.set_text(0, 'test')
var child1 = self.create_item(root)
child1.set_text(0, 'test-1')
var child2 = self.create_item(root)
child2.set_text(0, 'test-2')
var subchild1 = self.create_item(child1)
subchild1.set_text(0, "test-3")