Using percentage-based children of an HBox container
<?xml version="1.0"?> <mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"> <mx:HBox width="400"> <mx:Button label="Label 1" width="25%" /> <mx:Button label="Label 2" width="40%" /> <mx:Button label="Label 3" /> </mx:HBox> </mx:Application>