Calculating a distance between 2 movie clips?

I have two movie clips. One is simply on the screen, meaning it can be referred to as _root.movieclip1. The other is within another movie clip, like _root.movieclip2.movieclip3. How do I calculate the distance between the movieclips 1 and 3? The third's coordinates are shown not as they would be on stage, but from inside the second movie clip. I'm using Macromedia Flash Professional 8.