This book constitutes the refereed proceedings of the Third International Joint Conference on Discrete Geometry and Mathematical Morphology, DGMM 2024, held in Florence, Italy during April 15-18, 2024. The 34 full papers included in this book were carefully reviewed and selected from 51 submissions. They were organized in topical sections as follows: Digital Geometry - Models, Transforms, and Visualization; Computational Aspects of Discrete Structures and Tilings; Learning Based Morphology; Hierarchical and Graph-Based Models, Analysis and Segmentation; Discrete and Combinatorial Topology; and Mathematical Morphology and Digital Geometry for Applications.