Display List confusion (simple question)

programming flash
1

So I have:

var clip1:MovieClip = new MovieClip();
var clip2:MovieClip = new MovieClip();
var clip3:MovieClip = new MovieClip();

addChild(clip1);
clip1.addChild(clip2);

can anyone tell me why it won’t let me go:
clip1.clip2.addChild(clip3);

What’s the proper way to go about adding children to children?