Here is why you can share a neutral. The neutral current on a single phase panel for a multiwire branch circuit (2 hots and a neutral) is equal to the difference in the amp load of the cir. Thus if cir A has 10 amps and cir B has 20 amps and they share the neutral then the neutral will only see 10 amps.
This is why the neutral generally doesn't have to be larger than the circuit conductors. Here is the formula for 3 phase
The only potential problem with the application of your formula to the real world is that it is based on the assumption that all loads have a PF of 1.
In some (maybe artificial, but maybe practical) cases
other than those of non-linear loads and harmonic currents, the neutral current could be larger than what the formula shows.
In particular, if you have a resistive load on one phase, say A-N, and a highly inductive load on one of the other phases (B-N or C-N depending on my uncertainty about phase rotation
) then the neutral current could be greater than the formula states because the vector of the inductive load current is such that the geometric construction on which the formula is based does not apply.