The formula only changes slightly.

(Input voltage - LED1 Voltage - LED2 Voltage - LED3 Voltage)/LED current in amps = Resistor value ohms

So for three LEDs in series running at 2Volts each and a current of 10mA (0.01A) the resistor value will be (12-2-2-2)/0.01 = 600ohm. The nearest value that is easy to get is 620ohn and would be fine.

Make sure the input voltage is greater than the total of all the LED voltages.

Don't worry about voltage fluctuations. Most LEDs run happily from 5 - 20mA, if you calculate using 10mA any voltages changes in a car will not cause the current to go outside the 5 - 20ma range.

Rod.