This book constitutes the proceedings of the 20th International Conference on Grid, Cloud, and Cluster Computing, GCC 2024, the Third International Conference on Quantum Technologies, ICEQT 2024, and the 21st International Conference on Modeling, Simulation and Visualization Methods, MSV 2024, held as part of the 2024 World Congress in Computer Science, Computer Engineering and Applied Computing, in Las Vegas, USA, during July 22 to July 25, 2024. The proceedings include 15 papers from GCC and ICEQT 2024, which have been selected from a total of 63 submissions. For MSB 2024, the 5 papers included have been accepted from 26 submissions. They have been organized in topical sections as follows: Grid, cloud and cluster computing; emergent quantum technologies; and modeling, simulation and visualization methods.