E' sufficiente un solo timer per tutti gli mc.
codice:
mc1.visible=false
mc2.visible=false
mc3.visible=false
var tempo:Timer=new Timer(2000,0)//10 secondi
tempo.addEventListener(TimerEvent.TIMER, vai)
function vai(evt:TimerEvent):void{
    
    mc1.visible=false 
    mc2.visible=false
    mc3.visible=false
  tempo.stop()
 tempo.reset()
}
btn1.addEventListener(MouseEvent.CLICK, onClick1)
btn2.addEventListener(MouseEvent.CLICK, onClick2)
btn3.addEventListener(MouseEvent.CLICK, onClick3)
 
 function onClick1(evt:MouseEvent):void{
     mc1.visible=true
     tempo.start()
 }
 
 function onClick2(evt:MouseEvent):void{
     mc2.visible=true
     tempo.start()
 }
 function onClick3(evt:MouseEvent):void{
     mc3.visible=true
     tempo.start()
 }