For 3 VOLTS for the PIC I use an LM-317L 3 terminal adjustable regulator,
which is good for about 100 milliamperes.   I use a 220 ohm resistor between
the output terminal and the adjust terminal, and approx. 325 ohms from the
adjust terminal to ground. Use a capacitor from output to ground - several
microfarads tantalum will do.    If you want to adjust the voltage  +/- 10%,
use 270 ohns in series with a 100 ohm trimpot, from the adjust terminal to
ground.
I have used 3 volts on my PICDEM-1 board and get good results with 3v with
the PIC-16C71 chip.
Emerson Hoyt, Hoyt Instrument Lab, Beaverton, Oregon.