3S refers to the integration of security, safeguards, and safety concerns in nuclear installations. 

According to the IAEA Specific Safety Requirements (SSR-2/1):

"Safety measures, nuclear security measures and arrangements for the State system of accounting for, and control of, nuclear material for a nuclear power plant shall be designed and implemented in an integrated manner so that they do not compromise one another."

Additional details about 3S integration, including the challenges to implementation of 3S, can be found in "Interfaces among Safety, Security, and Safeguards (3S) - Conflicts and Synergies " by Sanders, Pope, Liu, and Shuler