var btns = [btn1, btn2, btn3]
for(var i:int=0; i<btns.length; i++)
{
    btns[i].i = i;
    btns[i].addEventListener(MouseEvent.CLICK, onClick);
}
trace(i);//2
function onClick(event:MouseEvent):void
{
    trace(i);//always 2
    trace(event.currentTarget.i);//0 or 1 or 2
}