Stereo Headset Volume Different
What you suggest is not the way to fix it.
Headphones typically have a nominal resistance anywhere between 8R and several hundreds of ohms (around 32R is common). Any wire/connector configuration should be much lower (if you measure with a meter you should see around 0.1R or 0.2R, most of which is the meter leads - it's good practice to check first by shorting them together).
The basic idea is that the wires and connectors used to hook things up have MUCH lower resistance than the load (the headphones in this case). It's generally so low that we call it zero. (As the wires are only a few meters and we are at AF we can neglect inductance and capacitance. This remains true unless we are discussing RF circuits which are different animal.)
Once in a very long while you might see a connector or wire which somehow goes high resistance (not open circuit) - like a few hundred or thousand ohms. Usually when this happens it's a solder joint, a dirty connector, or VERY occasionally a PCB track, and the resistance you see is not very stable, so you wouldn't ever try to fix it by adding resistors. The right way is always to find the bad connection and make it almost zero ohms again (clean the connector, resolder the joint).
The presence of 3 or 5 wires in the cable is not a big deal. The overall resistance still is damn close to zero. You don't have to take some cores out because the one you are replacing has a different number. Mostly this is about flexibility.
Yes, those enamel (if that's what the coating is) are hard to solder. Where I could I've often replaced them with ordinary twin-screened cable which is a bit heavier and less flexy but lasts longer and is easily replaced.