Use of Separate Neutral Cable

Although I have never seen an one core power cable, you can use anything what does not put to risk (yours and) other people's health and property.

Well yes, you can use single core conductors in a three phase systrem (which I have done once in a special situation) but.....

a) You have to be careful of inductive heating effects created by circulating currents when passing the cables through magnetic metal (such as steel), idealy by passing the cables through non-magnetic aluminium.

and

b) You need to check your local standards to see if it is allowed in your particular application.

Is the obvious solution impossible to implement?

For a test or other temporary installation, I do not see a problem. For any permanent installation if you do not the correct components, buy them.

Local code dictates. But the basic idea is, that a neutral have to be able to carry the full current of a phase in the case of a failure. A melted / scorched undersized neutral is no fun.

If it's star connected, why do you need neutral?

Or are your loads single phase?

As already mentioned, the problem will be when passing through a metal panel: the single neutral cable must pass in the same hole as the 3 core cable (even is the load is 3 phase, start connected, in case there was a possibility of the neutral carrying some current).

Also, the 3 core cable should not be armoured! the armour could become the host of unbalanced currents when the neutral start conducting some of it!

All this is necessary if you need the Neutral wire for phase to neutral loads !!