Try the following experiment:
Take three fixtures, connect them to a shared neutral and to three separate phase wires.
Using a true-RMS DVM measure the current in the common neutral.
Disconnect one fixture, take the measurement again.
Disconnect another fixture, take the measurement again.
The true-RMS meter should take into account any harmonics produced by the ballasts and show you the worst case loading.
With that information you can determine if the neutral needs to be over-sized (depending on the number of fixtures and the worst case balance).
:happyyes:It is my belief that in all cases a properly sized shared neutral will be less expensive to install and less expensive to operate than separate neutrals.
:happysad:The only negative is that electricians who install them and later maintain them need to understand them.