question:  Why were three Quest ships built?
context: [Transcriber's Note: This etext was produced from Planet Stories Fall 1949. In the last hours the star ahead had grown brighter by many magnitudes, and had changed its color from a dazzling blue through white to the normal yellow, of a GO sun. That was the Doppler effect as the star's radial velocity changed relative to the Quest III , as for forty hours the ship had decelerated. But now unwonted excitement possessed the hundred-odd members of the Quest III's crew. The great chamber, a full cross-section of the vessel, had been at first a fuel hold. At the voyage's beginning eighty per cent of the fifteen-hundred-foot cylinder had been engines and fuel; but as the immense stores were spent and the holds became radioactively safe, the crew had spread out from its original cramped quarters. He had already set up on the calculator a course that would carry them to Earth. As the Quest III's fuel stores and the hope of success in man's mightiest venture had dwindled, so the strength had gone out of him. Weeks and months had passed for the Quest III in interstellar flight while years and decades had raced by on the home world. If success comes there, a century will have passed before we can return to Earth; friends, relatives, all the generation that launched the Quest ships will be long since dead. "We face the unpleasant realization that what was feared is probably true—that worlds such as the Sun's are a rare accident, and that we may complete our search without finding even one new Earth. Certainly the building of this ship and its two sisters, the immense expenditure of time and labor and energy stores that went into them, left Earth's economy drained and exhausted. Only once in a long age does mankind rise to such a selfless and transcendent effort—the effort of Egypt that built the pyramids, or the war efforts of the nations in the last great conflicts of the twentieth century.
output:  To have a better chance of success if any failed.
options:  To have a better chance of success if any failed.  So each could travel for 20,000 years.  To transport more people to the new planet.  In case the others break down.