How much current or power do You need? If no more then 1,5A National made an universal ajastuble regulator, with minimum discrete elements. And You may get him for free as a sample. With good heatsink it can take up to 3A. http://www.national.com/ds/LM/LM117.pdf Theres a formula to calculate output voltage by using 2 resistors. For 12V input voltage and 3V output you need R1=240 Ohm and R2=330 Ohm.