Electrical – Driving 3w leds in a car

Your proposed circuit will not work. Let's say a single three LED string draws 20mA, then 24 strings in parallel will draw 24 × 20mA = 480mA.

Now take a look at the datasheet for a typical 555. Although the maximum output current for the 555 is not explicitly mentioned, the 'Low Output Voltage' parameter on page 3 is only spec'd up to 200mA. This means the 555 will not be able to higher sink currents than that.

What you need is a driver stage, a transistor that can be controlled by the 555 and can drive the required current. Simplest is to use a MOSFET, which is perfectly viable with a 12V supply voltage. Because you didn't mention the exact LED current, I can't propose a transistor part number, but the circuit looks like this:

^{simulate this circuit – Schematic created using CircuitLab}

Actually the below may work too, but I can't test it. The discharge pin is open collector and inverted from the output pin. The MOSFET again inverts the signal, hence the BJT from the circuit above can be eliminated.

^{simulate this circuit}

For selecting a proper transistor, look for I(D) being larger than the total expected LED current. Make sure you have some head room, maybe a factor 1.5 - 2 or more.

By decoding your question, this is what I arrive at:

You are using the two resistors as a voltage divider, with the LEDs wired in parallel connected to the junction of these two resistors.

^{simulate this circuit – Schematic created using CircuitLab}

This is not a valid approach, the voltage divider will not provide the 1:4 (approximately) voltage division your calculations indicate.

Firstly, either apply the three LEDs in series, with one resistor in series, or if you must have them in parallel, each will need a series resistor.

^{simulate this circuit}

The above indicative values of resistors are assuming 20 mA per LED as optimal. Do your own calculations for your specific LEDs.

For a ready to use online calculator, use an "LED resistance calculator" instead of a voltage divider one. This one even provides the correct schematic or wiring diagram as you prefer, to wire the LEDs up.