And your problem is? Are those three other mxml files components or
I think your structure like this:
<view:Comp1 />// Java.mxml, components stored in different files.
<view:Comp3 />// Flex.mxml.
On click of specific button you can set viewStack.selectedIndex = 0 // for Comp1 & so on...
If you are still facing problem of Component not yet initailize u can try creationPolicy="all" but which will be increase your application loading time.