B3 is a surprise too. The particles are prepared to be up spin in the beginning. In classical mechanics, measurements should not change states. In quantum mechanics, it collapses superposition no matter how gentle it is. Being a right spin has a 50% chance to be in up-spin or down-spin when measured. i.e. why B3 have half and half particles exits in different paths. To answer it quantitatively, we can create a math model for that.