This is the non-catalytic component of the active enzyme, which catalyzes the hydrolysis of ATP coupled with the exchange of Na(+) and K(+) ions across the plasma membrane. The exact function of the beta-3 subunit is not known. The sodium/potassium-transporting ATPase is composed of a catalytic alpha subunit, an auxiliary non-catalytic beta subunit and an additional regulatory subunit. Interacts with catalytic alpha subunit ATP12A. Identified by mass spectrometry in melanosome fractions from stage I to stage IV. The C-terminal lobe folds into an immunoglobulin-like domain and may mediate cell adhesion properties. Belongs to the X(+)/potassium ATPases subunit beta family.