My three are Andrew Humphreys, John Gibbon, Horatio Wright and Charles Griffin. Add Sheridan as a cavalry corps commander. Those four are the reason Grant was able to trap Lee at Appomattox. Sheridan and Wright smashed Early in the Valley. Wright successfully organized and led one of the largest assaults of the war (Petersburg). Humphreys, Gibbon and Griffin were experienced battlefield commanders who, by the end of the war, by far outclassed anyone the Confederates had.