[Cornford]

s1.visible = true;
s2.visible = true;
s3.visible = false
s1.addEventListener(MouseEvent.MOUSE_DOWN, s1Down);
s1.addEventListener(MouseEvent.MOUSE_UP, s1Up);
addEventListener(Event.ENTER_FRAME, frameEntered);
function s1Down(e:Event):void{
    s1.startDrag();  
}
function s1Up(e:Event):void{
    s1.stopDrag();
}
function frameEntered(e:Event):void{
    if (hitTest(s1, s2)){
    s1.visible = false;
    s3.visible = true;
    }
}

например как-нибудь так