The growing use of 3D concrete printing has increased the need for cementitious mixtures that combine adequate mechanical performance with lower environmental impacts and production costs. However, these requirements may conflict, making mixture selection a multi-criteria problem. This study assessed nine cementitious mixtures for 3D concrete printing by integrating Life Cycle Assessment (LCA) with the MARS-SC multi-criteria decision-making method. We adopted a cradle-to-gate system boundary and conducted the environmental assessment according to ISO 14040 and ISO 14044 using SimaPro and the Ecoinvent database. We integrated environmental, functional, and economic indicators using the Methodology for the Relative Sustainability Assessment of Building Technologies (MARS-SC) to obtain an overall sustainability score. The mixture containing 40% limestone filler and 10% metakaolin showed the best environmental performance, with a global warming potential of 358 kg CO₂ eq/m³. The mixture reduced the GWP by 45.01% and 54.28% compared with two mixtures with higher cement contents and without partial cement replacement. However, this environmental advantage did not result in the highest overall sustainability score because of lower functional and economic performance. When the three dimensions were considered together, Blf30 and Blf40 achieved the highest sustainability scores (NS = 0.66). Although both mixtures had lower mechanical performance than the cement-rich mixtures, their environmental and economic results led to a more favorable overall assessment. The integrated analysis therefore shows that reducing environmental impacts does not, by itself, necessarily produce the most sustainable mix.