LED in strings of 4, or use resistors

My leds require a restitor at anything over 3V. I put an resistor on each postive coming off of each led. Arn't they all still getting 12V? If i ran a 12V from the front of my car to the back wouldn't i still have 12V at the back even if i have 20 things running off of it? Maybe thats a question to your question. isn't it constant 12V all the way throught the wire. so i think you would need a resistor at each led. But Im only 15 and i havn't learned much about 12V and wiring yet But i know im right about the leds.

This is what I am thinking

In a series configuration each device will draw a portion of the voltage, therefore if you have a 12vdc power supply and 4 3vdc leds wired in series each one would see 3vdc.  However, you do need to worry about current.

LEDs are not resistive devices, they are considered discreet components.  If you try to measure the resistance of an LED you'll run in to issues (the same issues you run in to when measuring the resistance of a diode).  Therefore, if you wire 4 LEDs in series with NO resistor you will have nothing to limit the load, and that can be a very bad thing.  It wouldn't take much to destory your chain of LEDs, especially considering that if one goes out the entire chain will go out.

I would recomend calculation exactly how much current you think you'll need, and pick a resistor to add inline with the LEDs.  Use one LED for the entire chain. 

this is a neat little site I found, it'll calculate the size of resistor you need

http://led.linear1.org/1led.wiz

When wiring groups of LEDS in series, I found that every-other LED was dimmer.  so it looked spotty. 

It's better to get the right value of resistor for each LED.