This book constitutes the refereed proceedings from the Third International Workshop on Foundation Models for General Medical AI, MedAGI 2025, held in conjunction with the 28th International Conference on Medical Image Computing and Computer Assisted Intervention, MICCAI 2025, in Daejeon, South Korea, during September 27, 2025.The 19 full papers included in this book were carefully reviewed and selected from 31 submissions. The workshop aims to foster discussions that accelerate the transition from task-specific medical AI systems toward more generalized frameworks capable of addressing diverse tasks, datasets, and domains.