This book constitutes the refereed proceedings of the Third International Conference on HCI for Cybersecurity, Privacy and Trust, HCI-CPT 2021, held as part of the 23rd International Conference, HCI International 2021, which took place virtually in July 2021.

The total of 1276 papers and 241 posters included in the 39 HCII 2021 proceedings volumes was carefully reviewed and selected from 5222 submissions.

HCI-CPT 2021 includes a total of 30 papers; they were organized in topical sections named: usable security; security and privacy by design; user behavior analysis in cybersecurity; and security and privacy awareness.