6 12V to 3V buck converter

There are many ways of converting 12V to 3V. You can use regulator IC, transistors, zener diodes and many more. The problem of these converters is very low efficiency and most of their power is converted to heat. A dc-dc converter is a need in this case.
The circuit below is a simple buck converter that converts a 12V DC (usually from battery) to 3V DC. The heart of the circuit is the 555 timer IC that controls the Mosfet and the entire circuit. This converter can deliver a current up to 5A at a 3V load. Current can be made even higher if Q1, D1, and L1 are replaced with much higher current capacity.

buck converter diagram

To make the output voltage more regulated, you can add the circuit below across the converter output.

regulator circuit

Part List:
R1 - 5 kilo ohms 1/4W
R2 - 2.7 kilo ohms 1/4W
R3 - 1.2 kilo ohms 1/4W
R4 -  just remove (its a dummy load)
C1, C2 - 100 nano-Farad capacitor
C3 - 2200uF electrolytic capacitor rated 16V
Q1 - IRF9530 or any similar P-channel Mosfet (10A current min)
D1 - SR506 or any diode rated 6A min (SCHOTTKY preferable)
L1 - 1mH Inductor minimum 6A current rating
555 - LM555 IC
Regulator circuit (optional)
      Dz - 1N5223B or 2.7V zener diode min 1/2W
      R - 120 ohms 1W
      Q - 2N3055, TIP41C or similar NPN transistor

555 pins

2N2222 pins

IRF9530 or PMOS pins