How can I show a new Fragment within a single tab?

I am creating a three tabs that contains one fragment each now I want to replace the first Tab's Fragment with a new Fragment within the same Tab. How can I do that with the tab remaining the same?

My Code -

Tab_Widget.java

    public class Tab_Widget extends TabActivity {

            TabHost tabHost; 

        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.tab_host);

            tabHost = getTabHost();

            tabHost.addTab(tabHost.newTabSpec("Tab1")
                .setIndicator("Tab1")
                .setContent(new Intent().setClass(this, main_activity.class)));

            tabHost.addTab(tabHost.newTabSpec("Tab2")
                .setIndicator("Tab2")
                .setContent(new Intent().setClass(this, second_activity.class)));

            tabHost.addTab(tabHost.newTabSpec("Tab3")
                    .setIndicator("Tab3")
                    .setContent(new Intent().setClass(this, third_activity.class)));

            tabHost.setCurrentTab(0);
        }
     }

main_activity.java

public class main_activity extends FragmentActivity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.mainfragment);
    }
}

main_fragment.java

public class main_fragment extends Fragment
{
    LinearLayout mLayout;
    Button mButton;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {

        mLayout = (LinearLayout) inflater.inflate(R.layout.main_view, null);
        mButton = (Button) mLayout.findViewById(R.id.btn);
        mButton.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {

                Fragment mFragment = new third_fragment_view();
                FragmentTransaction ft = getFragmentManager().beginTransaction();
//              ft.replace(R.id.Maincontainer, mFragment);
                ft.replace(R.id.main_fragment, mFragment);
                ft.addToBackStack(null);
                ft.commit();
            }
        });
        return mLayout;
    }
}

My XML

mainfragment.xml

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/Maincontainer"
android:layout_width="match_parent"
android:layout_height="match_parent">

<fragment
  android:name="com.fragment.main_fragment"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:id="@+id/main_fragment">
</fragment>
</LinearLayout>

By, this the Fragment is getting called no doubt, but its overlapping the previous Fragment.