Android - ayuda con notificaciones

public class NotificationService extends FirebaseMessagingService {

    public static final String TAG = "FIREBASE";

    private int ONGOING_NOTIFICATION_ID;

    @Override

    public void onMessageReceived(RemoteMessage remoteMessage) {

        //super.onMessageReceived(remoteMessge);

        Log.d(TAG, "From: " + remoteMessage.getFrom());

        Log.d(TAG, "Notification Message Body: " + remoteMessage.getNotification().getBody());

        Intent i = new Intent(this, Activity_Tres.class);

        PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, i, PendingIntent.FLAG_ONE_SHOT);

        Uri sonido = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);

        NotificationCompat.Builder builder = new NotificationCompat.Builder(this)

                .setSmallIcon(R.drawable.logo_in)

                .setContentTitle("Notification")

                .setContentText(remoteMessage.getNotification().getBody())

                .setAutoCancel(true)

                .setSound(sonido)

                .setContentIntent(pendingIntent);

        NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);

       notificationManager.notify(0, builder.build());

    }

}

manifest

    <uses-permission android:name="android.permission.INTERNET" />

    <application

        android:allowBackup="true"

        android:icon="@mipmap/ic_launcher"

        android:label="@string/app_name"

        android:roundIcon="@mipmap/ic_launcher_round"

        android:supportsRtl="true"

        android:theme="@style/AppTheme">

        <activity android:name=".Activity_Uno">

            <intent-filter>

                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />

            </intent-filter>

        </activity>

        <activity

            android:name=".Activity_Dos"

            android:label="@string/title_a"

            android:parentActivityName=".Activity_Uno"

            android:theme="@style/AppTheme.NoActionBar">

            <meta-data

                android:name="android.support.PARENT_ACTIVITY"

                android:theme="@style/AppTheme.NoActionBar"

                android:value=".Activity_Uno" />

        </activity>

        <activity

            android:name=".Activity_Tres"

            android:label="@string/title_b"

            android:parentActivityName=".Activity_Uno"

            android:theme="@style/AppTheme.NoActionBar">

            <meta-data

                android:name="android.support.PARENT_ACTIVITY"

                android:theme="@style/AppTheme.NoActionBar"

                android:value=".Activity_Uno" />

        </activity>

        <service

            android:name=".NotificationService">

            <intent-filter>

                <action android:name="com.google.firebase.MESSAGING_EVENT"/>

            </intent-filter>

        </service>

    </application>

</manifest>