This two volume set LNCS 10602 and LNCS 10603 constitutes the thoroughly refereed post-conference proceedings of the Third International Conference on Cloud Computing and Security, ICCCS 2017, held in Nanjing, China, in June 2017.   The 116 full papers and 11 short papers of these volumes were carefully  reviewed and selected from 391 submissions. The papers are organized in  topical sections such as: information hiding; cloud computing;  IOT applications; information security; multimedia applications; optimization  and classification.