Making a TS cable with a 3-conductor wire
What I'd probably do is to use the shield for one connection (ring) and the put the two inner conductors in parallel for the tip connection.

It costs you nothing to use both inner conductors except for stripping and soldering two wires instead of one.  It gets you lower resistance, which might be needed in a long cable.  It also gets you a little redundancy - if one wire breaks the other is still connected.