This book constitutes revised selected papers from the Third International MICCAI Brainlesion Workshop, Brain Les 2017, as well as the International Multimodal Brain Tumor Segmentation, Bra TS, and White Matter Hyperintensities, WMH, segmentation challenges, which were held jointly at the Medical Image computing for Computer Assisted Intervention Conference, MICCAI, in Quebec City, Canada, in September 2017. The 40 papers presented in this volume were carefully reviewed and selected from 46 submissions. They were organized in topical sections named: brain lesion image analysis; brain tumor image segmentation; and ischemic stroke lesion image segmentation.