The sensitivity and the initial offset of voltage between the collectors of the two-collector lateral bipolar magnetotransistor of npn-type with the base, formed in the well being the third collector (3KBMTBW) at the reduced rate of the surface recombination in the base, have been experimentally investigated. The magnetotransistor switch-on circuit determines the operation mode and the device parameters. It has been shown, that the magnetosensitivity of voltage achieves 11 V/T.