Here's a better, quicker, cheaper way, that doesn't require ANY parts. In fact, you'll have parts left over. You say you need to drop 12VDC, down to 3VDC, correct ??
12VDC divided by 1.5VDC batteries = 8 batteries. In order to get 3VDC, just remove 6 of the batteries, which will leave you with 2 batteries. 2 batteries @ 1.5VDC each = 3VDC Simple as pie & you have 6 batteries left over, & didn't have to spend a cent. Well, it looks good on paper anyway. LOL.............rob
Use a voltage divider. A pot will allow you to fine-tune the voltage, or you can just use two resistors, with one value being thrice the value of the other. I recommend 3kohms for R1 and 1kohms for R2.
Depending on current required at 3V, you could use 1117 3.3 as mentioned, or use LM317 (1.5A), or LM350 (for up to 3.0A). Heatsink it well, you should have room in a spiro, linear regulators turn excess voltage into heat ... Schematics available online and are also in the datasheet for the regulator.