You can go through the math and see that there is no need to include the neutral in a total current derating of a conduit. The maximum power transfer without exceeding a current X is when all three phases carry the maximum current, X. In this case, the neutral carries zero current. But what about unbalanced loads? The simplest case is one phase carrying the maximum X and the others zero. In this case the total current is 2X, less than the maximum 3X. If two phases carry the maximum and the other zero, the neutral also carries X and the total is 3X, equal to that from the max load. Any other combination also produces a total current of 3X or less.
There is an exception. Very nonlinear loads like power supplies in computers. They usually draw only during the voltage peak and the neutral current often does not cancel completely. I've heard stories of how the neutral has burned out when supplying a data center full of computers with
3 phase.