Final answer:
The third basic principle of decontamination after 'get it off' and 'keep it off' is to 'contain it'. This step restricts the spread of contaminants, protecting people and the environment.
Step-by-step explanation:
The basic principles of decontamination are get it off, keep it off, and contain it. The third principle involves ensuring that contaminants are not allowed to spread and cause further harm, a process which may involve physical containment or the use of barriers. This essential step in the decontamination process helps to limit exposure and prevents the contaminants from affecting more people, the environment, or spreading to other areas.