How to replace fragment C with fragment A when back button is pressed?

Theory

Use the addToBackStack(tag: String): FragmentTransaction method from within the FragmentTransaction in order to mark a point where you want to return to. This method returns the FragmentTransaction instance for chain-ability only.

Then Return with the popBackStackImmediate(tag: String, flag: int): void method from the FragmentManager. The tag is what you specified before. The flag is either the constant POP_BACK_STACK_INCLUSIVE to include the transaction marked or 0.

Example

What follows is an example with the following layout having a FrameLayout with id content_frame where the fragments are loaded into.

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

<TextView
    android:id="@+id/textView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/hello_world" />

<FrameLayout
    android:id="@+id/content_frame"
    android:layout_below="@id/textView"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

</RelativeLayout>

The code below marks a fragment by it's fragment class name when replacing the content of the layout element with id content_frame.

public void loadFragment(final Fragment fragment) {
    
    // create a transaction for transition here
    final FragmentTransaction transaction = getSupportFragmentManager()
            .beginTransaction();
            
    // put the fragment in place
    transaction.replace(R.id.content_frame, fragment);

    // this is the part that will cause a fragment to be added to backstack,
    // this way we can return to it at any time using this tag
    transaction.addToBackStack(fragment.getClass().getName());

    transaction.commit();
}

And to complete this example a method that allows you to get back to that exact same fragment using the tag when you loaded it.

public void backToFragment(final Fragment fragment) {

    // go back to something that was added to the backstack
    getSupportFragmentManager().popBackStackImmediate(
            fragment.getClass().getName(), 0);
    // use 0 or the below constant as flag parameter
    // FragmentManager.POP_BACK_STACK_INCLUSIVE);

}

When implementing this for real you might want to add a null check on the fragment parameter ;-).

You need to do 2 things - name the FragmentTransaction from A->B and then override onBackPressed() in your containing activity to call FragmentManager#popBackStack (String name, int flags) when you are on Fragment C. Example:

Transition from A->B

getSupportFragmentManager()
  .beginTransaction()
  .replace(R.id.container, new FragmentB(), "FragmentB")
  .addToBackStack("A_B_TAG")
  .commit();

Transition from B->C will use a similar transaction with "FragmentC" as its tag.

Then in your containing Activity override onBackPressed():

@Override
public void onBackPressed() {
  if (getSupportFragmentManager().findFragmentByTag("FragmentC") != null) {
    // I'm viewing Fragment C
    getSupportFragmentManager().popBackStack("A_B_TAG",
      FragmentManager.POP_BACK_STACK_INCLUSIVE);
  } else {
    super.onBackPressed();
  }
}