james:
are you sure your motor can run in 3v ????.what kind of dc motor is that!, if you trying to slow down the rpm rate,cut down your dc voltage in half may not work ,dc motor have there limitation in power.
howerever that's said your right about operating voltage, with only 100ma load a LM317 + two transistor+ two i/o pin , you can have 12v , 6v & 3v in control, chech out the LM317 spec then you'll see what i said
good luck & have fun
Chris Chang