Think of it this way: assume you have a balanced 3 phase wye load and the currents are 10 amps at 0 degrees, 120 degrees, and 240 degrees. Add the 10 amps at 0 and 120 degrees and you get 10 amps at 60 degrees, which is 180 degrees from the 240 degree load and that adds to 0 amps in the neutral. So the highest load on the neutral can not be higher than any of the phases. Nothing wrong with a shared neutral circuit.
