Anyone who knows me knows I love these things.
Now, I have a question. This will work, electrically, and is safe, but I can't figure out if there's a rule against it.
You already know I can take two 14awg hot conductors, each on a 15A breaker but on opposite legs, and share one 14awg neutral between them. The neutral currents from the two circuits cancel each other out, and the maximum current through the neutral wire is 15A.
I can't add a third circuit, because two of them will be on the same leg, and their neutral currents would add up instead of cancelling. This means the maximum current through the neutral wire would be 30A...
... so why the heck can't I just upsize the neutral to accomodate? ie, shove three 14AWG hots and one 10AWG neutral down a conduit?