First Arduino Project - Need Advice (powering WS2812B LEDs)
Powering the LED panels
Powering your panels will be your greatest challenge!
These LED panels are powered by a single +5V power input per panel and do not need a resistor in the power supply path (they actually need a very low resitance path). You do need a good quality +12V DC -&gt; +5V DC power supply (or multiple, as explained below).
Since 3 x 256 RGB LEDs require a large amount of power when they're all lighted at the same time (something like 60mA * 768 =~ 46A), the power supply should be able to accomodate that. It might be better to use three individual, smaller (~15A) power supplies. You could opt for power supplies smaller than 15A, but you should ensure that all LEDs will never light up at full intensity at the same time.
For supplying power from a 12V car battery, you could use high quality 12V DC to 5V DC buck converters (they're efficient). Use good quality cable with a diameter large enough for transporting the large currents necessary. If you need connections in the path of the power supply, make sure they are good quality as well. To reduce losses in long stretches of wire, you should place the buck converter(s) near the LED panels.
Connecting the data lines
The color and intensity of Individual LEDs is controlled via the data input coming from your Arduino and it only needs one input to control all 768 LEDs. The first panel's &quot;data out&quot; should be connected to the next panel's &quot;data in&quot; and so on, like your drawing.
Software
I recommend you look up the FastLED library and some of its examples to get familiar with controlling large amounts of addressable RGB LEDs. There are many good examples and interesting projects out there.