This volume contains a record of some of the lectures and seminars delivered at the Third International School on Engineering Trustworthy Software Systems (SETSS 2017), held in April 2017 at Southwest University in Chongqing, China. The six contributions included in this volume provide an overview of leading-edge research in methods and tools for use in computer system engineering. They have been distilled from six original courses delivered at the school on topics such as: rely/guarantee thinking; Hoare-style specification and verification of object-oriented programs with JML; logic, specification, verification, and interactive proof; software model checking with Automizer; writing programs and proofs; engineering self-adaptive software-intensive systems; and with an additional contribution on the challenges for formal semantic description. The material is useful for postgraduate students, researchers, academics, and industrial engineers, who are interested in the theory and practice of methods and tools for the design and programming of trustworthy software systems. .