Let's take a purely resistive "full boat" and say each phase conductor is loaded to 20 amps, if we tally these conductors currents we will have 60 amps flowing and the neutral would be at zero.
Now take one of the phases and reduce it to 15 amps, when we tally the conductors again we will have 55 amps flowing and the neutral will have 5 unbalanced amps flowing so once again a total of 60 amps when all is added, so the neutral does not add to total current unlike a grounded conductor does in a two wire circuits.
Note; In heavy non-linear loads this would not necessarily be true.
Roger
