If you know the resistance of your load you can determine the voltage that would be necessary to dissipate 3 watts.
Power law tells us watts=voltage * current Ohm's law tells us voltage=current * resistance
Sub "watts/voltage" from power law for "current" in Ohm's law and we have
Multply both sides by voltage to get them on one side:
Take the square root of both sides (too hard to do the radical in ascii):
So, if you know how resistive your load you can now calculate the necessary voltage to dissipate 3 watts.
Well if your plugged into a house hold outlet and in the US your pulling ~120volts. The whole US runs off of 120volts. There are a few devices that run off a 240 volt setup in the US (things the generate heat) but they really run off 2 - 120 volt lines. But with 3 watts your not using one of those :P.
If your working of a battery system, well it depends on the battery.
The equation is voltage=watts/AMPS. I can't really help because I don't have all the numbers. Or if there is another way to figure it out, I don't remember. It's been a few years since I learned that...
In simple terms P=VI where P represents "Power", V=volts and I = Current