Driving 3w leds in a car
You are best to use some sort of Switchmode Current source .If you use parallel linear regs you will get an efficiency no better than the filament lamps that you replaced.One option is to boost up to 16V with all the leds in series .This keeps current down making the DCDC convertor cheap and easy to build .The LED currents are equal despite spreads in the individual terminal voltages .You can protect your series string from LED open circuit failure by using an amplified Zener arrangement .If you buy a driver from ebay you will learn nothing .