The voltage from a car battery can be anything from 11V to 15V.
If you use just a resistor, you must be sure the current through the LEDs will never be too high at the maximum voltage.
However, with a 9V LED assembly, that means the resistor may be dropping anything between 6V and 2V; the current will vary by a factor of three times over the battery voltage range, from maximum at 15V down to one third at 11V.
The three ohm resistor would give around 2A at 15V (engine running and battery at full charge) down to ~0.67A with the battery near flat.
That's why an electronic regulator is better, if you want constant brightness.