Fermented beverage containing probiotics can have interesting biological properties enhanced by fermentation in addition to the benefits promoted by the ingestion of these microorganisms. However, the viability of probiotics in alternative vegetable matrices is still a challenge. Germination, for example, is a process that can favor the growth of probiotics and positively impact the food properties. Thus, this work aimed to determine the most adequate composition of grains for fermentation by the probiotic yeast Saccharomyces boulardii. For this, the impacts of germination and fermentation on the microorganism growth and on the antioxidant and antidiabetic potential of the fermented beverages were evaluated. A simplex centroid mixture design was applied, in which the composition of the fermentation medium was performed using isolate, binary or ternary mixtures of germinated and non-germinated lentils, brown rice and sorghum. The samples were analyzed for DPPH- and ABTS-radical scavenging activities, ferric antioxidant power (FRAP), and α-amylase enzyme inhibition assays. The germination favored the yeast growth (109 - 1012 CFU/mL), but this effect was not observed for biological activities. Considering all the results, and the influence of the processes, the binary mixture of lentils and brown rice was the best combination to obtain a fermented beverage with higher antioxidant properties (ABTS - 1286.50 µmol TE mL-1; DPPH - 470.85 µmol TE mL-1; FRAP - 370.27 µmol TE mL-1) and antidiabetic potential (46.50% α-amylase inhibition) as well as probiotic growth. This condition was also chosen for further studies in the development of a functional beverage with probiotics in a vegetable matrix that can enable the consumption of these products among the vegan public.