How do you reduce12 volt DC to 3 volt DC?

Answer 1

With some difficulty. You can use a resistor in series to drop the voltage, or you can use a zener diode. If you want to be elegant you get a DC-to-DC converter and set it up to do the job for you.

A series resistor will work only if the load current doesn't vary.

Answer 2

A: At that low voltage a supposed to be zener could be a series of diodes. Anyhow you must first determine the load at 3.5 volts then add 5 to 10 ma for the zener for total current. a resistor must be placed in series with the zener and its value would be 12v -3.5v divided by total current.