Ancillary protein that functions as a regulatory subunit of the voltage-gated potassium (Kv) channel complex composed of pore- forming and potassium-conducting alpha subunits and of regulatory beta subunits. KCNE3 beta subunit modulates the gating kinetics and enhances stability of the channel complex (PubMed:10646604, PubMed:11207363, PubMed:12954870). Alters the gating of the delayed rectifier Kv channel containing KCNB1 alpha subunit (PubMed:12954870). Associates with KCNC4/Kv3.4 alpha subunit to form the subthreshold Kv channel in skeletal muscle and to establish the resting membrane potential (RMP) in muscle cells (PubMed:11207363). Association with KCNQ1/KCLQT1 alpha subunit may form the intestinal cAMP-stimulated potassium channel involved in chloride secretion that produces a current with nearly instantaneous activation with a linear current-voltage relationship (By similarity). {ECO:0000250|UniProtKB:Q9JJV7, ECO:0000269|PubMed:10646604, ECO:0000269|PubMed:11207363, ECO:0000269|PubMed:12954870}. This is the function of Ensembl gene identifier ENSG00000175538 (KCNE3, potassium voltage-gated channel subfamily E regulatory subunit 3).