All you need is 300.3(B). That may well be violation enough. But since you have PVC conduit, you need to follow the lead to 300.20(B) and beyond, to see if your installation is in compliance.
But as to hazards, the magnetic fields generated in each wire of a 3-phase, 4-wire system will tend to cancel each other out, if they are all in the same conduit. That will prevent any adverse influence on the conduit itself, or on nearby metal items.
With phase conductors in one conduit and the neutral in another, every thing will still be cancelled out, provided the loads are balanced. But that will not always be true. In the extreme case, one phase will be unloaded, the loads will be carried by the other two, and the neutral current will be the same as the phase currents. That will create a significant magnetic field in the one conduit that contains the neutral wire, since that conduit has no other wires whose currents might cancel out the field created by the neutral. If the conduit is magnetically permeable (e.g., steel), this will induce a large current in the conduit itself. Possible results include overheating and maybe a fire, and a shock hazard to anyone touching the conduit.
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