This book constitutes the thoroughly refereed post conference papers of the Third International Conference on Blockchain and Trustworthy Systems, Blocksys 2021, held in Guangzhou, China, in August 2021.*The 38 full papers and the 12 short papers were carefully reviewed and selected from 98 submissions. The papers are organized in topical sections: Contents Blockchain and Data Mining; Performance Optimization of Blockchain; Blockchain Security and Privacy; Theories and Algorithms for Blockchain; Blockchain and Internet of Things; Blockchain and Smart Contracts; Blockchain Services and Applications; Trustworthy System Development.*