When somebody seems to be having trouble grasping the idea of the neutral carrying no current in a balanced 3-phase system, I find that it sometimes helps to start by explaining a 3-wire DC system, where the positive and negative voltages are more easily visualized, then go on to a 3-w 1-ph AC system. (The latter should be that much easier for most Americans to follow due to the widespread use of the system for residential services.)
Once someone can visualize what is happening with the unbalanced current from two hot legs 180 degrees out of phase, it then becomes easier to extend the principle to 3 phase and 120 dgrees.
