Power depends upon if you run a single LED wirh correct resistor from 12 Volts, or 3 LED's in series from 12 Volts with a series R.
1. If you put 3 single LED's drawing 20 mA each, with their own resistor on the 12 Volts supply the power usage will be higher.
2. Put the 3 LED's in series, drawing 20 mA with a single dropping resistor, usage will be lower.
In the first case each resistor will have to dissipate about 8 to 9 volts at 20 mA in heat. (taken 3½ to 4 volts per white LED)
In the second case the LED's consume about 10.5 volts and the series resistor only has to dissipate about 1.5 - 2 volts at 20 mA's.