Small GTP-binding protein that plays a central role in regulated exocytosis and secretion. Controls the recruitment, tethering and docking of secretory vesicles to the plasma membrane (PubMed:11598194). Upon stimulation, switches to its active GTP-bound form, cycles to vesicles and recruits effectors such as RIMS1, RIMS2, Rabphilin-3A/RPH3A, RPH3AL or SYTL4 to help the docking of vesicules onto the plasma membrane (By similarity). Upon GTP hydrolysis by GTPase-activating protein, dissociates from the vesicle membrane allowing the exocytosis to proceed (By similarity). Stimulates insulin secretion through interaction with RIMS2 isoform RIMS2 and RPH3AL effectors in pancreatic beta cells (PubMed:15159548, PubMed:20674857). Regulates calcium-dependent lysosome exocytosis and plasma membrane repair (PMR) via the interaction with 2 effectors, SYTL4 and myosin-9/MYH9 (By similarity). Acts as a positive regulator of acrosome content secretion in sperm cells by interacting with RIMS1 (By similarity). Plays a role in the regulation of dopamine release by interacting with synaptotagmin I/SYT (By similarity). Interacts with RIMS1 and RIMS2 (PubMed:11056535, PubMed:11431472, PubMed:12401793, PubMed:12578829). Interacts with Rabphilin-3A/RPH3A and Rab effector Noc2/RPH3AL (PubMed:12578829). Interacts with SYTL4 (PubMed:12590134). Interacts with RAB3IP. Interacts with SGSM1 and SGSM3 (PubMed:17509819). Interacts with SYT1 (By similarity). Interacts with MYH9; this interaction is essential for lysosome exocytosis and plasma membrane repair (By similarity). Interacts with STXBP1; this interaction promotes RAB3A dissociation from the vesicle membrane (By similarity). Interacts with SNCA (By similarity). Interacts with GDI1, GDI2, CHM and CHML; phosphorylation at Thr-86 disrupts these interactions (By similarity). Interacts with MADD (via uDENN domain); the GTP-bound form is preferred for interaction (PubMed:18849981). Cycles between a vesicle-associated GTP-bound form and a cytosolic GDP-bound form. Expressed in the forebrain and hippocampus (at protein level) (PubMed:31651360). Found in active zone in brain nerve terminals (PubMed:11598194). Phosphorylation of Thr-86 in the switch II region by LRRK2 prevents the association of RAB regulatory proteins, including CHM, CHML and RAB GDP dissociation inhibitors GDI1 and GDI2. RAB3-deficient mice show an incomplete and slow secretion response in brain nerve terminals after exhaustive stimulation. The replenishment of docked vesicles after exhaustive stimulation is also impaired. Belongs to the small GTPase superfamily. Rab family.