how resize the Qwidget objects within a window

[ChrisW67]

If B and C inherit class A, the typical meaning of derived, then the question makes no sense. B is an A and C is an A. This has nothing to do with the number of windows displayed on screen or communication between them.

I suspect you have three completely independent windows, A, B, and C, showing some widgets. You want events in A to trigger actions in B or C. You need to read about signals and slots