In this example I will show you how to add a movieClip to your flash AS3 as you would in AS2 which use the attachMovie function.
var testClip:MovieClip = new clips();
testClip.x = 200;
testClip.y = 200;
and then do this:
Get to properties of the movie clip by right clicking at the item at your library. Give the movieClip a name and click at the 'export for actionScript checkbox.
The class name will be generated automatically for you.
Your done. You may test your movie..
Take note that "var testClip:MovieClip = new clips();" the 'clips()' are actually the class name as in the properties box.