As long as you as are asking for a "hack", then yes: simply add another div to your HTML, with :before
and :after
pseudo-elements. Move the border-bottom to these elements and give them widths that look right.
Of course, this requires an extra HTML element with no semantic value -- though wrapping all three of those content divs in another would make sense, and you could add the properties to that.
Nested or multiple pseudo-elements, if they become supported, would be a better way.
Unfortunately, just adding a bottom-border to the :before and :after styles won't work (they will still extend too far), because of the way CSS borders are rendered -- to see this make the borders thicker and of different colors. The answers to this question also illustrate this.