Swapping INTERNAL Movie Clips

I’ve gotten so caught up into loading external swf’s that I have completely bypassed being able to call INTERNAL movie clips (movie clips within the library rather than external).
Quick Example:
I have a movie named shapes.swf. It has 3 buttons labeled Box, Star, and Triangle. Within the same shapes.swf, I have 3 (internal) movie clips: a box, a star, and a triangle.
All I want to do is to be able to assign the buttons to show these shapes on the screen. And to be able to have the shapes “swap” when a different button is pressed. I know this is soooo easy, but it has just slipped my mind completely somehow. If someone could simply put in the correct code, I would greatly appreciate it! I’m sure it would only take about 10 seconds!