This should do it:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            import mx.events.NumericStepperEvent;

            private var nsMask:String = "000";

            private function formatNumber(evt:Event):void {
                var value:String = numericStepper.value.toString();
                value = (nsMask + value).substr(-nsMask.length);
                numericStepper.mx_internal::inputField.text = value;
            }
        ]]>
    </mx:Script>

    <mx:NumericStepper id="numericStepper"
            minimum="0"
            maximum="255"
            value="3"
            width="100"
            textAlign="center"
            creationComplete="formatNumber(event);"
            change="formatNumber(event);" />

</mx:Application>

Peter