B3 or B-3 may refer to: Vitamin B3. B3, an international standard paper size (353×500 mm), defined in ISO 216. B3, a four cylinder engine made by Mazda B3 is the IATA code for Bellview Airlines Keystone B-3, a United States Air Force bomber. HMS B3, a B-class submarine of the Royal Navy. USS B-3, a B-class submarine of the United States Navy. B3 was a German boy band.