What is the required resistor for 3 (1 watt each) LEDs in series with a source voltage 12 volt ? ?

Are they 3 volt or 3.5 volt?

One watt LED's are very bright and need a heat sink. Since you only want to light your computer, maybe better to use four 5mm white LED's in series and not resistor required.

you pick the resistors in sequence with the LED's. you want to apply separate resistors. you could not use one large resistor because the voltage drop by potential of the LED's are not the very similar. some should be brighter and some dimmer or no longer on in any respect with a unmarried resistor. i ought to bypass for a modern-day of about 20 mA. you also want to appreciate that the 12 volts on your automobile isn't precisely 12 volts. The alternator desires to enhance the voltage to about 13.8 to real value the battery. Use 15 volts to be conservative in you calculations. the nearest established fee resistor is 680 ohms for extra or less 20 mA of modern-day.