WASHINGTON—President Donald Trump signed three executive orders making it easier for the federal government to fire employees it considers to be poor performers, the White House said Friday, drawing rebukes from union representatives who said the sweeping changes were a “direct assault” on the rights of millions of workers.

The orders reduce the length of time that an employee can be retained using a “performance improvement period,” currently ranging from 60 to 120 days depending on the agency, encourage agencies to fire...