buttonBar.as class code:-

package custocumponent
{
   import flash.events.MouseEvent;
   
   import mx.controls.Alert;
   import mx.controls.Button;
   import mx.controls.ButtonBar;

    public class buttonBar extends ButtonBar {
         
        public function buttonBar() {
               super();
               var mybutton1:Button = new Button();
               mybutton1.label = "Submit";
               this.addChild(mybutton1);
                
               var mybutton2:Button = new Button();
               mybutton2.label = "Login";
               this.addChild(mybutton2);
               
               var mybutton3:Button = new Button();
               mybutton3.label = "Register";
               this.addChild(mybutton3);                                          
        }
    }

}


mainapplication.mxml code:-


<?xml version="1.0"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
    xmlns:MyComp="custocumponent.*">    
    <MyComp:buttonBar/>
</mx:Application>