Most PICs can 'read' or input 3V logic, no problem. The high threshold is 2.1V or something. Read the very end of the PIC datasheet, it will give you logic level high minimums for different pins.
The problem is outputting a 5V high into a 3V device. You run the *risk* of frying inputs on the 3V device. Couple things to consider:
Use a voltage divider. If you put 5V into a voltage divider of 3/5, you of course get 3V out. 0V in, 0V out. But this requires two resistors - not too bad.
You could put an inline resistor inbetween devices. Say 100k or so would limit the current to 20uA (5V to 3V is a 2V drop over 100k is 20uA) being injected into the 3V device. Kinda shady, but it works for bread boards.
Or you could go with the really ghetto implementation and hook the 5V device directly to the 3V device. Many 3V devices are 5V tolerant so they can handle 5V signals. The manufacturer may not say it is 'so' but it may work fine. Other devices are *very* sensitive and must have a nice 3V level logic interface. The worst that could happen is that you fry your 3V device - fair warning.