Regulatory subunit of the voltage-gated potassium (Kv) channels composed of pore-forming and potassium-conducting alpha subunits and of regulatory beta subunit (PubMed:9857044). The beta- 3/KCNAB3 subunit may mediate closure of potassium channels (By similarity). Increases inactivation of Kv1.5/KCNA5 alpha subunit- containing channels (PubMed:9857044). May display nicotinamide adenine dinucleotide phosphate (NADPH)-dependent aldoketoreductase activity (By similarity). The binding of oxidized and reduced NADP(H) cofactors may be required for the regulation of potassium channel activity (By similarity). {ECO:0000250|UniProtKB:P62483, ECO:0000250|UniProtKB:P63144, ECO:0000269|PubMed:9857044}. This is the function of ENSG00000170049 (KCNAB3, potassium voltage-gated channel subfamily A regulatory beta subunit 3).