Final answer:
The three domain extensions not used by the military are .usn, .usa, and .edu.
Step-by-step explanation:
The three domain extensions that are not used by the military are .usn, .usa, and .edu. The .usn extension is not commonly used and is not associated with any particular organization. The .usa extension can be misleading as it is not specific to the military but represents the United States as a whole. The .edu domain extension is used by educational institutions such as universities and colleges.
Learn more about Domain extensions for the military