It is with great pleasure to welcome you to The 3rd ACM International Symposium on Blockchain and Secure Critical Infrastructure (BSCI 2021). Under extenuating circumstances due to Covid-19, BSCI 2021 will be fully virtual, instead of the planned June 7-11, 2021, in Hong Kong, China. Despite the additional challenges due to the pandemic, this symposium still serve as a platform to share and discuss state-of-the-art research advances on blockchain and secure critical infrastructure, ranging from security and privacy issues in blockchain to attack methods on blockchain to securing the smart grid, etc. As part of the discussion, we hope to foster new, cutting-edge research collaboration between researchers and practitioners with complementary interests and background. This is the third symposium organized as part of ACM AsiaCCS, which builds on success of the prior editions, 2019 (New Zealand) and 2020 (Taipei, China). In this year, we attracted a total of 34 submissions from Asia, Australia, Europe, and the United States. All submissions were subject to rigorous reviews by subject matter experts, and eventually only six (6) submissions were accepted to appear in the symposium as regular papers (i.e. acceptance rate is 17.65%). Six (6) submissions were accepted as short papers. We are also honored to have Dr. Samee U. Khan from Mississippi State University as our keynote speaker, who will be speaking on "Blockchains, Security, and Infrastructures: What we Know and What we Can Know".