This book constitutes the proceedings of the Third International Conference on Human and Artificial Rationalities, HAR 2024, which took place in Paris, France, in September 2024. The 19 full papers and 4 short papers included in this book were carefully reviewed and selected from 52 submissions. They are organized under the following topical sections: Artificial reasoning and models; Moral reasoning; Artificial intelligence and cognition; Rationality and Dual Process; Reasoning and special needs; Education; Experimental procedures in cognition. * This book also contains one additional paper from the HAR 2023 conference.