Definition of "three-line" [three-line]

• A trigram; a system of 3 coplanar straights. (noun)

The Century Dictionary (Public Domain)

• "His ministers pressed for focus on the crisis as they insisted that the government will impose a so-called "three-line whip" requiring full lawmaker support against a vote in Parliament tomorrow seeking a referendum on U.K. membership of the EU."
• "The commentators Robert Lee and Ross McFarlane, presumably under a three-line whip to talk up the joys of 3D, made frequent reference to the beauty of what we were seeing."
• "Although some informed readers may look at these three-line poems and assume they are haiku, I do not call them haiku, because they are more personal and more earthy."