Use transistor to control 12v LEDs, schematic help needed

Just need a MOSFET with a low turn on voltage: http://www.digikey.com/product-detail/en/AO3416/785-1011-1-ND/1855953

The reason for using 3 leds in series is so that you can power all 3 from a single resistor basically, thats how the 12v strips work anyway ..

That and it's easier than individually soldering them all with their own resistors.

et say that the led-board contains 4x 3v leds in series, then i need 12v to get them to their maximum brightness right? Why would i need a current limiting device like a resistor?

You always need some sort of current limiting device with an LED because the forward voltage drop changes with temperature, age of the device and from device to device. So there is never one right consistent voltage to use. Read http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

cjdelphi: The reason for using 3 leds in series is so that you can power all 3 from a single resistor basically, thats how the 12v strips work anyway ..

That and it's easier than individually soldering them all with their own resistors.

That's right.