Big Bloom is not a standalone fertilizer (0-0.5-0.7) and is considered a supplement (provides vitamins, amino acids, natural growth hormones, enzymes, and beneficial organic microbes). It's designed to be used throughout the grow and lots of people use it this way. It's all organic and helps with root growth (important for young autoflowering strains) as well as bacterial health in the soil.
As these plants are just breaking 3 weeks old and are in a mostly Fox Farm Ocean Forest mix, only a minimal amount of Grow Big is needed at this stage (if any really) as the soil still has plenty of nutrients available for the plants.