I forgot to mention that in G+B the available half elevator being one side of longitudinal axis when applied imparts rotary motion. In alternate law side stick commands g which indirectly deflects the elevator it's difficult to accurately anticipate this effect. By moving to direct law pilot directly moves elevator so is in better position to judge and control this torsional effect. That's why it's recommended to move to direct law early. Actually even in G+Y also, flap3 is not important because you only have slat which doesn't have 3 position but getting speed to Vapp before gear is important because that positions the elevator for Vapp with stick neutral.