The rise of new processor hardware extensions that permit fine-grained and flexible trusted execution, such as Intel's SGX, ARM's TrustZone, or AMD's SEV, introduces numerous novel challenges and opportunities for developers of secure applications. There is a burning need for cross-cutting systems support of such Trusted Execution Environments (TEEs) that spans all the layers of the software stack, from the OS through runtime to compilers and programming models. The 3rd Workshop on System Software for Trusted Execution (SysTEX) will focus on systems research challenges related to TEEs, and explore new ideas and strategies for the implementation of trustworthy systems with TEEs. The workshop is also open to papers exploring attacks on current TEEs and strategies for mitigating such attacks.