Control 12V LED Strip with 5V from RaspberryPi or Arduino
For RGB strip you indeed need 3 pcs transistors or MOSFET's. Most commonly RGB LED strips have "common anode", so they take in 12V from one wire, and 3 other wires need to be connected to ground make each color glow.
So connect the power to 12V and take 3 Logic level N-channel MOSFET's (http://www.bgmicro.com/fet.aspx), put the MOSFET's between the R, G and B wire and ground. Then connect the MOSFET's gates to Raspberry GPIO pins and control the colours with PWM.
Use pigpio, or WiringPi (https://github.com/wiringPi) to generate the three PWM signals.