Although a simple way and probably not how they did it you could take three images all static and use stop action scripts for each one. Then use action scripts to tell the frame to load on mouse over. THis would change the picture each time you rolled over them. Once you did this you could extend the timeline and make each slide into place. Basic idea is to tell the flash to play a frame when you mouse over the text and add a stop script after the frame(s) are loaded.

I'm sure what they are doing is more complexe becuase it has a certin effect based on what is already selected, I don't know that their way adds antyhing becuase it slides in from different directions depending on the current image.

Hope this help,
Jason