Remote control led has 3 volt power source output I need it to switch on a 12 volt power source to power led array
Bruno's circuit uses a FET, which can also be used, but I'll give you an answer for the BJT (Biploar Junction Transistor) too.  



First the NPN, at the left. You supply 3 V to base resistor R2. Since the base will be at 0.7 V (the base-emitter junction acts as a diode) you'll have a base current of 2.3 mA. The transistor's collector current will be a multiple of that. For a general purpose transistor the current gain (\$h_{FE}\$) is typically around 100, but will decrease for a transistor in saturation. The 2.3 mA base current will be enough to give you 20 mA collector current though.  

R1 is needed to limit the LED current, you'll always need it if you supply LEDs with a voltage source. The value depends on the LEDs' voltgae drop. I've calculated with 2 V per LED, then for 2 LEDs you have 8 V across the resistor. Apply Ohm's Law: R = V/I, then for 20 mA you want a 390 &Omega; resistor. (actually 400 &Omega; but 390 is a common E12 value, while 400 isn't).

For the FET's R3 the same holds, so also 390 &Omega;. A FET is controlled by voltage instead of current, and you'll need a logic level gate FET to be sure it switches on at 3 V. R4 is not always required, but limits the short current peak during switching; the gate forms a capacitance and microcontrollers and other logic doesn't like capacitive loads. R5 prevents that the gate would be floating if it wouldn't be driven.