How do I target multiple buttons

programming flash
1 

for(var i:Number = 0; i < 3; i++) {
var mc:MovieClip = this.attachMovie("myButton", "myButton" + i, i);
mc.index = i;
mc.onPress = function():Void {
trace(this.index);
//current selected _alpha=50. how to control other buttons _alpha at the sametime.
this._alpha =50;
}
}

I have a question about the code.
Now. I can contorl the current selected mc.like myButton1.But How I can control the other button action at the sametime.like myButton1._alpha =50, myButton2,myButton3,myButton4…_alpha = 30.

thanks a lot!