How to use "11(no.s)" 3v 20mA LED on a 12v input headlight on my car?

replace the headlamp? no can do. an automotive headlamp has to meet specific SAE requirements to be street legal. an important requirement is the beam shape, so that it does not blind on-coming traffic. the beam also has to generate a certain FC of illumination at a distance, i don't remember the spec but 11 LEDs at 0.06 watts each won't replace the 55 watt sealed beam lamps for brightness. the input for your car is not 12 volts. The battery voltage is 15.5 volts when charging and 13.5 volts with the engine off. if you design for 15.5 volts to keep the LEDs from burning out, then they won't light at 13.5 volts.