If the load is balanced and will remain balanced then there is not need to count the neutral as a CCC, but how can you assume a balanced load when you have 3 individual plug circuits that can be used by one, two or three circuits at any given time. If it were just a 3-phase load, I would not count the neutral as a CCC.
Rich, consider this example of linear loads.
In a perfectly balanced wye 20 amp MWBC loaded 100 percent there will be a total of 60 amps flowing in the circuit, 20 amps flowing in A, B, and C phases, the neutral has 0 amps flowing.
Turn off any one phase and the neutral will carry the same as the other two phases for a total of 60 amps, no change.
Mix any odd number into the equation with any or all of the three phases and the circuit will still only have a total of 60 amps flowing, no change again.
There is no reason in most situations to have to count a neutral as a CCC.
Roger