Код AS3:
var t1:TextField = (btn_1.upState as DisplayObjectContainer).getChildAt(1) as TextField;
var t2:TextField = (btn_1.downState as DisplayObjectContainer).getChildAt(1) as TextField;
var t3:TextField = (btn_1.overState as DisplayObjectContainer).getChildAt(1) as TextField;
//жирным шрифтом
t1.htmlText = t2.htmlText = t3.htmlText = "<b>Кнопка";