AS3 startDrag/stopDrag is similar to AS2 except it uses mouse events to detect the mouse movement, below is a simple implementation:

?View Code ACTIONSCRIPT
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package {
 
    import flash.display.*;
    import flash.events.*;
 
    public class DragExample extends Sprite {
 
         function DragExample() {
                 addEventListener(MouseEventType.MOUSE_DOWN, onMouseDown);
                 stage.addEventListener(MouseEventType.MOUSE_UP, onMouseUp);
         }
 
         private function onMouseDown(event:Event):Void {
                 this.startDrag();
         }
 
         private function onMouseUp(event:Event):Void {
                 this.stopDrag();
         }
    }
}

You will need stage to listen to mouse up event in order to stop dragging as you might release your mouse too quick or not on the sprite.