The Complete Zarathustra Trilogy

When the star Zarathustra went nova, the desperate survivors packed themselves into spaceships and fled in all directions. Habitable worlds were few, and colonizing a new planet requires much more than just settling a newly discovered island of Old Earth. New planets were different in thousands of ways, and the differences could mean disaster. Brutal environment, poor leadership, and little equipment caused most to revert to a non-technical subsistence farming economy, or worse. After hundreds of years the Zarathustra Refugee Planets were either forgotten or in quarantine. Ripe for exploitation.

Polymath (1974) aka Castaways' World (1963)
The Avengers of Carrig (1969) aka Secret Agent of Terra (1962)
The Repairmen of Cyclops (1965)