Bonaldi, AnnaHartley, PhilippaBraun, RobertPurser, SimonAcharya, AnshumanAhn, KyungjinAparicio Resco, MiguelBait, OmkarBianco, MicheleLópez Caniego, MarcosLópez-Caniego Alcarria, Marcos2025-10-072025-10-072025Bonaldi, A., Hartley, P., Braun, R., Purser, S., Acharya, A., Ahn, K., Aparicio Resco, M., Bait, O., Bianco, M., Chakraborty, A., Chapman, E., Chatterjee, S., Chege, K., Chen, H., Chen, X., Chen, Z., Conaboy, L., Cruz, M., Darriba, L., … Zhou, X. (2025). Square Kilometre Array Science Data Challenge 3a: Foreground removal for an EoR experiment. Monthly Notices of the Royal Astronomical Society, 543(2), 1092-1119. https://doi.org/10.1093/mnras/staf14660035-8711https://hdl.handle.net/11268/16362We present and analyse the results of the Science Data Challenge 3a (SDC3a, https://sdc3.skao.int/challenges/foregrounds), an epoch of reionization (EoR) foreground removal exercise organized by the Square Kilometre Array Observatory (SKAO) on SKA simulated data. The challenge ran for 8 months, from 2023 March to October. Participants were provided with realistic simulations of SKA-Low data between 106 and 196 MHz, including foreground contamination from extragalactic and Galactic emission, instrumental, and systematic effects. The challenge results provide a means for all teams to understand and improve their performance. This challenge indicates that the comparison between independent pipelines could be a powerful tool to assess residual biases and improve error estimation.engAttribution 4.0 Internationalhttp://creativecommons.org/licenses/by/4.0/journal article10.1093/mnras/staf1466open accessCiencias del espacioAnálisis de datosInteligencia artificialGoal 9: Build resilient infrastructure, promote sustainable industrialization and foster innovationGoal 13: Take urgent action to combat climate change and its impactsGoal 17: Partnerships