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 the

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.