This is nice, but it's not always the case that +3 indent is the right solution (e.g. if I'm copying already indented code it may be over indented).
It's basically a non problem in most other languages, and a IDE formatter hook will always clean up the code and organize it correctly in a way that you cannot get in Python.
Have you not used any of such IDEs/plugins?
It's not X+3 indent, it's "starting at +3", so if you have lines with +10 indent (overindented) copied and paste them at +3 indent, they all get indents cut by 7 levels and end up at the same +3 level as expected.
It's basically a non problem in most other languages, and a IDE formatter hook will always clean up the code and organize it correctly in a way that you cannot get in Python.