Randy, look at it mathematically: Start with an equal load, say 20 amps, on all three phases. The neutral current is the sum of the three currents, including the phase angles, which equals 0.
Now, remove the load on one phase, as well as its contribution to the neutral sum, -20 amps from each. This results in 20 amps subtracted from the previously-balanced zero neutral current, meaning 20 amps.
So that leaves 20 amps on two phases, and 20 amps on the neutral. Now, let's "check our math" by reversing the process: Each amp drawn by the third phase reduces the neutral current by the same amount, until it's back to zero.
Fine Electric Co.