Well you are trying to nest a menu inside an item tag in your main.xml
Try this instead:
<item android:id="@+id/action_settings"
android:showAsAction="always"
android:title="@string/action_settings"
android:orderInCategory="100"
app:showAsAction="never"/>
<item android:id="@+id/action_b"
android:id="@+id/help"
android:title="@string/help"
app:showAsAction="never"/>
<item android:id="@+id/action_c"
android:id="@+id/about"
android:title="@string/about"
app:showAsAction="never"/>