You can use a simple series resistor alon, as suggested by
@kellys_eye.
Or you can use one of the step-down modules you linked to in post #3 to step down to e.g. 5 V, then again use a series resistor to limit the current. This will be more energy efficient than a series resistor to 12 V, but more costly.
Your decision.
Which LEDs are you going to use? 5 V LED sounds like the LED has an integrated current source, then you wouldn't need a series resistor.
3 V LED sound more like a typical white LED with a pass voltage of 3 V at the rated current. But this does not mean you can connect the LED to a 3 V voltage source and expect the rated current to flow. You need to control (or limit) the current.
LEDs operate different from incandescent lamps which you can simply connect to the appropriate voltage source. Please read the resource I linked in post #2.