3D concrete printing (3DCP) is a significant advancement in modern construction, enabling tailored rheological and mechanical properties for additive manufacturing. Optimal flowability, static yield strength, and mechanical performance are essential for successful 3DCP, ensuring workability during extrusion and stability for subsequent layers. Incorporating sustainable materials like fly ash enhances the environmental appeal of 3DCP, making it a promising green alternative to traditional methods. This study examines the impact of key mix components on the rheological and mechanical properties of 3DCP to develop optimized compositions balancing flowability and stability. A factorial design approach was used to systematically vary cement content (550–650 kg/m3), fly ash (10–20% of cement), superplasticizer (0.2–1.0 kg/m3), and water (295–315 kg/m3). All mixes made with 3 kg/m3 of sisal fibres. This method improves optimization efficiency by identifying effective combinations and reducing experimental trials. Rheological properties were evaluated using slump flow, cone penetration, and cylindrical slump tests, while compressive and flexural strength tests assessed mechanical performance. Statistical models identified the water and binder content as the most critical factor influencing rheology. Fly ash improved fluidity and strength in conventional concrete but had limited benefits in 3DCP due to water absorption. Superplasticizers enhanced fluidity and reduced water use but required careful balancing to prevent adverse effects. The factorial design further refined these findings, efficiently identifying parameter interactions critical for achieving optimal 3DCP performance.