Audio transformer: What to do with an unused winding?
Option 3 is best

It does not matter whether you connect the primaries together or not, you are still stuck with the primary voltage appearing on pins 3 and 4 as a result of transformer action, and still stuck with that interacting with whatever parasitic capacitance exists.

At higher frequencies, the leakage inductance will tend to decouple winding 34 from 12, so the parasitic capacitance on 34 would be better controlled by connecting it directly to 12. This favours option 3 over option 2 somewhat.

Option 1 does not do what you want. The 150ohm resistor will appear in parallel with the transformed 600ohm load, and present a 75ohm load to your primary.