I may be wrong, but this looks like a case for nested BOM’s
To be more specific…
If you have 3 independent wires in a single cable AND you manufacture the 3 wires as well as the entire cable, THEN you create a BOM for each individual wire (includes all materials for just one of the component wires).
Once you have all of the internal cable components using their own BOM’s THEN you can create a final BOM for the manufacture of the consolidated cable.
The components to make the cable will be the finished goods of the 3 different wire builds.
When it is time to build the cable, call up the BOM for the finished cable and in the exploded view ti will contain all of the the information of the component wire BOM’s as well as any new raw materials needed to complete the cable.
So, I am not sure if I understood the problem you are facing but it sounded like this might be the solution. Hope it helps.