Help please!

Please can someone tell whats wrong with this.

I have 3 movieclips and a button on the stage.

I have this in frame 1 of an actions layer and when you click the button the 3 movieclips are supposed to play. The trace outputs the correct instance name (box1, box2, box3)

but1.onRelease = function() {

    for (y=1; y<=3; y++) {
            mc = "box"+y;


thanks for any help.