ball1.scaleX=ball1.scaleY = 0.6+Math.random() * 0.3;
ball2.scaleX=ball2.scaleY = 0.6+Math.random() * 0.3;
ball3.scaleX=ball3.scaleY = 0.6+Math.random() * 0.3;

ball1.addEventListener(Event.ENTER_FRAME, onEnterframe);
ball2.addEventListener(Event.ENTER_FRAME, onEnterframe);
ball3.addEventListener(Event.ENTER_FRAME, onEnterframe);

function onEnterframe(e:Event): void {
var mc:MovieClip = e.target as MovieClip;

if ((mc.y - mc.height/2)> stage.stageHeight){
mc.y =- mc.height/2;
}else {
mc.y+=mc.height * 0.2;
}
}