 Why were three Quest ships built?
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. 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. 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.
 To have a better chance of success if any failed.
 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.