Engineering education has seen a growing interest in how students regulate their learning as a group in interdisciplinary projects. This study adds to the current literature on social regulation of learning by conducting a comparative case study of three interdisciplinary group projects addressing real-world challenges. Semi-structured qualitative interviews were synthesised into narrative episodes representing key aspects of the groups' regulative behaviours. We found indications of co- and socially shared regulation across all groups, with noteworthy differences in the project phases that led to varying student experiences. We discuss key factors that affected regulation along four themes we identified: (1) goal setting and planning, (2) implementation, monitoring, and evaluation, (3) the role of supervisors, and (4) the impact of disciplines. We offer insights for practitioners and provide a foundation for future research on social regulation in interdisciplinary group learning.