I normally keep an old spare phone, with my latest music playlist on it, in my Model S, as my main music source. Will this cause trouble with the M3’s phone as fob system?
No.
You can "pair" multiple phones with the car, similar to how you can BT-pair multiple phones to most other cars these days.
Then, you can pick which one is actively connected via BT to your Model 3. That would be the phone that will be used for music streaming and phone calls.
Tesla, like most other OEMs, supports only one active BT connection to the car at a time.
So if your old spare phone doesn't have a SIM in it, you will be able to stream music, but wont be able to make or receive calls via the car until you change the "active pairing" via the BT symbol at the top-right-hand corner of the screen.
HTH,
a
P.S.: Even without a SIM, your old phone can be used a Model 3 key as long as you have Tesla app installed, and Bluetooth is turned ON.