Yes the N currents will balance at the point of linking, that is one of the points of my argument, however until then they will sum.
I'm gona re read the OP. if the supply to the switch fuse is 3ph, and the loads from the fuse are single phase, then I don't see a problem. It's not conventional to wire seperate circuits for the same switchfuse, but it isn't dangerous.
As long as the cables are suitable for the protective devices, and they are suitable for the load, then the neutral only needs to equal the size of the largest phase conductor (not that of ever size them differently anyway).
Each circuit from the switchfuse will have to have it's own neutral, but the 3ph submain would only need one neutral. So in other words, the switchfuse becomes an unconventional DB.
Sorry to say Paul but if I understand the OP correctly then you're wrong. Let's say for example that there is a 20A load plugged into each of the three phases (60 total current). The three line conductors are out of phase with each other, therefore the resulting current in the neutral from any one of these lines will be out of phase also. In very simple terms; at the point in which L1 is drawing its 20A, L2 and L3 aren't drawing, therefore the neutral current is 20A, 120 degrees (150th of a second) later L2 is drawing 20A whilst L3 and L1 are drawing nowt. Neutral current = 20A. And so on...
If you didn't have blinkers on you'd understand mate.
We are discussing 3 single phase circuits with a shared N