Modulates channel gating kinetics. Causes unique persistent sodium currents. Inactivates the sodium channel opening more slowly than the subunit beta-1. Its association with NFASC may target the sodium channels to the nodes of Ranvier of developing axons and retain these channels at the nodes in mature myelinated axons. The voltage-sensitive sodium channel consists of an ion conducting pore forming alpha-subunit regulated by one or more beta-1, beta-2, beta-3 and/or beta-4 subunits. Beta-1 and beta-3 are non-covalently associated with alpha, while beta-2 and beta-4 are covalently linked by disulfide bonds. Beta-1 or beta-3 subunits associate with NFASC. Associates with SCN10A. Expressed broadly in neurons in the central and peripheral nervous systems, but not in glia and most non-neuronal cells. Weak detection in lung and adrenal gland. Belongs to the sodium channel auxiliary subunit SCN3B (TC 8.A.17) family.