To Research, Evaluate, Assess, and Treat Astronauts Act or the TREAT Astronauts Act (Sec. 3) This bill authorizes the National Aeronautics and Space Administration (NASA) to provide for: the medical monitoring and diagnosis of a former U.S. government astronaut or former payload specialist for conditions that NASA considers potentially associated with human space flight; and the treatment of such an astronaut or payload specialist for conditions that NASA considers associated with human space flight, including scientific and medical tests for psychological and medical conditions. The bill requires that such medical monitoring, diagnosis, or treatment shall be provided without any deductible, copayment, or other cost sharing obligation. NASA may not require such an astronaut or payload specialist to participate in the medical monitoring, diagnosis, or treatment authorized by this bill. NASA shall protect the privacy of all medical records generated with respect to such medical monitoring, diagnosis, and treatment and accessible to NASA. The term "government astronaut" does not include individuals who are international partner astronauts. NASA shall arrange with an independent external organization to undertake an independent cost estimate of the cost to NASA and the federal government for the implementation and administration of the activities of NASA under this bill. NASA shall study any potential privacy or legal issues related to the possible sharing beyond the federal government of data acquired under the activities of NASA. The NASA Inspector General shall periodically audit or review NASA's activities to prevent waste, fraud, and abuse.