This book constitutes the refereed proceedings of the Third International Conference on HCI in Mobility, Transport, and Automotive Systems, MobiTAS 2021, held as part of the 23rd International Conference, HCI International 2021, held as a virtual event, 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.MobiTAS 2021 includes a total of 39 papers which focus on topics related to urban mobility, cooperative and automated mobility, UX in intelligent transportation systems, and mobility for diverse target user groups.