Creating a simple voltage regulator for medium currents
I can do a little bit math for you. If you have Vin = 12VDC, Vout = 3.3VDC and draws I = 3A at 3.3V, then you will have (12 - 3.3) * 3 or 26.1W wasted power with a linear regulator. You will get totaly 9.9W, so with other words a efficiency of 37%.

The most efficiency buck-converter (Switched-mode) will only waste under 10% of the input power. Here is a simple example:



Since you only need 3A, then you set the Rsense to 0.01 Ohm, and to get 3.3VDC out of it, you need to change Rselect to 1.66K.