maybe QStackedWidget is what you are looking for.
Yes. Add the QStackedWidget to your vertical layout, then add the three sub-widgets as pages in the stack. Assign a stack page index (0, 1, or 2) to each of your buttons. So when the button for "a" is clicked, you set the current stack page to 0, for example. If you want a situation where -none- of the three sub-widgets is visible, then just add a plain QWidget instance as one of the pages.