If I understand what you are saying, this should be pretty easy. You just need to issue the appropriate push and pop screens:

MainScreenA:

...
UiApplication.getApplication().pushScreen(new MainScreenB));
...

And then:

...
UiApplication uiApp = UiApplication.getUiApplication();
UiApp.popScreen(uiApp.getActiveScreen());
...

There is no need for invokeLater() since (I'm guessing) you are in the UI thread.

The one caveat is if you want to close MainScreenB when you close MainScreenC.