How to clear fragment back stack?

Question

3 fragments:

• FragmentA: root fragment (implementing IRootFragment), opening by BottomNavigationView;
• FragmentB: opening from A-fragment by button;
• FragmentC: opening from B-fragment by button;

I can go to A-fragment from C-fragment by BottomNavigationView and i have to clear back stack, but dont know, why it's not working. When I moved from the first fragment to the second and press the button back, the fragments overlap each other and only after that it exits the application.

If i go C -> B -> A by back button and press the back button, all works good.

Code for opening fragments:

  fragmentManager
                .beginTransaction()
                .replace(R.id.fl_container, new FragmentA())               
                .commit();
  fragmentManager
                .beginTransaction()
                .replace(R.id.fl_container, new FragmentB())
                .addToBackStack(null)               
                .commit();
  fragmentManager
                .beginTransaction()
                .replace(R.id.fl_container, new FragmentC())  
                .addToBackStack(null)             
                .commit();

BackPressed:

 Fragment fragment = fragmentManager.findFragmentById(R.id.fl_container);
        if (fragment instanceof IRootFragment) {        
            if (fragmentManager.getBackStackEntryCount() > 0) {
                getSupportFragmentManager().popBackStack(                     
                fragmentManager.getBackStackEntryAt(0).getId(),
             FragmentManager.POP_BACK_STACK_INCLUSIVE);
            }
            super.onBackPressed();
        } else {
            fragmentManager.popBackStackImmediate();
        }

I'm trying all of this: Is this the right way to clean-up Fragment back stack when leaving a deeply nested stack?

fragmentManager.getBackStackEntryCount() always > 0