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The best way is to use an LED driver IC. You should be able to find constant current drivers out there that will supply 3 LED's in series with the right current. Simple resistors will work, but again I'd use 3 LED's in series to avoid having to throw all that power away in resistors completely. With resistors though the lights will get dimmer/brighter depending on weather you're running straight off the battery or if the alternator is running, which assumes this is for a car.
Use 3 3.5 volt LED's in series. That's 10.5 volts of diode drop from the LED's right there, good place to start. Obviously you'd need to do them in three's so you'd need 18 LED's 3X6, or 15 for 3X5. So you would need either 5 or 6 drivers (you may be able to find a multiple driver output IC)
I hate all these car applications that use 3.5 volt LEDs and a resistor to drop the rest, the entire point of LED's is they're efficient, if you add a big series resistor to control current you're throwing the heat away yourself, might as well just use a normal incandescent bulb!
You did ask for the 'best' way not the easiest =) If your have free air cooling your resistors will likely work forever if they're rated properly. Just keep mind mind how much power you're throwing away. If you're okay with that, the the circuit is fine the way it is.
The one thing I can say for using a single LED with a bigger resistor is that the larger resistor dropping more voltage will act more like a constant current source than the three LED's in series with the smaller resistor, it will dim/brighten more with voltage changes. That's why I recommended a constant current driver instead of a resistor as you get perfect lightning regardless of the voltage then.