V1 is referring to your output voltage in your equation and V2 will be referring to your input voltage. So V1 will be 3 V, your desired voltage and V2 will be 12 V, the initial voltage from your power supply.

The 9 V value you are using is not part of this problem, I assume that you have tried 12 V - 3 V = 9 V. This is not useful in this calculation and is useful in learning only to know that this is not the right equation to use for a voltage divider. Try to think of it in terms of multiplication and division, not addition or subtraction, it's voltage divider not a voltage subtractor.

Hope this helps.