Hi. Here's what I would suggest you can try:

right_btn.onRelease = function () {

var myTween:Tween = new Tween ( right3_mc, "_alpha", Regular.easeOut, 0, 100, 12, false );

new Tween ( right3_mc, "_alpha", Regular.easeOut, 100, 0, 12, false );

myTween.onMotionFinished = function () {

nextFrame ();

}

}

What I've done is I've created a variable with the Tween data type called myTween. The Tween class has an event handler method called onMotionFinished which is called when the Tween is completed. You can find out more details in the help section under Components Language Reference > Tween class. There is a list of other event handlers there.

Hope that helps. Cheers!