You can use -keep-generated-actionscript option. Right click on project, in Properties>Flex Compiler add "-keep-generated-actionscript" into Additional Compiler Arguments.
Select any tag within your MXML module, or for that matter select the module's root tag like <mx:Application. Then press your F3 function button. The Actiion script for for you MXML tag will be displayed in a new Flex Builder window.
Thanks for your replies.
Could you please let me know where genarated AS files will reside in Flex builder ?.