12v to 3v

You won't need anything if you put the LED's in series. When in series, the current (amperage) that flows through each LED is same whereas the voltage gets divided. So if you have 12volts and 4 LEDs then each LED gets 12/4 which is 3 volts. However if you put them in parallel(meaning every LED's + touching every other LED's + and every LED's - touching every other LED's -), they all get the full 12v but the current/amperage gets divided. To prove this theory, you see "Christmas lights" that are connected directly to a 110v supply without any circuits. Same principle. They have either 10 12volt lights in series (120 volts) or 50 3volt lights in series (needing 150 volts). LED's are more sensitive than cheap "Christmas bulbs" so the tolerance is less however you get the idea.

yes John, you're right. However if you use LEDs in series to any device for a voltage drop, you are more than likely running the risk of burning out your LED. These LEDs take very little current (about 7 to 20 milliamps). If your device that is needing the voltage drop is a discman for example, it will need about 300 mA and 3V, thereby frying your LEDs since it will get 3 volts but 300 mA across the LED, thereby burning it out. Check the link that you posted before, it has some details.