You need to better explain what you are trying to do?
Overall, yes depending on your application. Use a small switching transistor to drive a relay or larger transistor but again this is vague as you don't provide any information. How much current do you have available on your 3 volt source (as in where does it come from) and how much current at 12 volts are you looking to switch?
The easy answer is to use a opto coupler, bacisally it consists of a led on the 3 volt side and a photo transistor on the 12 volt side isolated by a beam of light all in a single package.
As Reloadron asked we need to know more data to give advice to exactly what will suit your circuit needs.