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 (By similarity). 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 (By similarity). Belongs to the sodium channel auxiliary subunit SCN3B (TC 8.A.17) family.