Pore-forming subunit of Nav1.3, a voltage-gated sodium (Nav) channel that directly mediates the depolarizing phase of action potentials in excitable membranes. Navs, also called VGSCs (voltage- gated sodium channels) or VDSCs (voltage-dependent sodium channels), operate by switching between closed and open conformations depending on the voltage difference across the membrane. In the open conformation they allow Na(+) ions to selectively pass through the pore, along their electrochemical gradient. The influx of Na+ ions provokes membrane depolarization, initiating the propagation of electrical signals throughout cells and tissues (PubMed:24157691, PubMed:28235671, PubMed:29466837, PubMed:35277491). In some secretory cell types, it also participates in cell excitability through membrane depolarization and regulates cells responsiveness to stimuli triggering secretion. For instance, it controls the release of serotonin/5-hydroxytryptamine by enterochromaffin cells and is required for both glucagon- and glucose- induced insulin secretion in pancreatic endocrine cells (By similarity). {ECO:0000250|UniProtKB:A2ASI5, ECO:0000269|PubMed:24157691, ECO:0000269|PubMed:28235671, ECO:0000269|PubMed:29466837, ECO:0000269|PubMed:35277491}. This is the function of ENSG00000153253 (SCN3A, sodium voltage-gated channel alpha subunit 3).