chris from brussels,
Although it would be very helpful, the menu bar of a VI is not available from the VI propeties. A workaround would be that each VI writes its menubar refnum onto an indicator of its front panel and then you can use the VI method "Get Control Value" to obtain the refnum with a VI reference.
However I suggest that each VI A,B,C polls itself if it is frontmost and if it is, writes its menubar refnum to some global control or otherwise notify the "menu checker" of its menubar refnum.