I have a Tabbar, with 3 Windows. Each Window has a TabBarItem and a NavLeftItem.
The NavLeftItem inside a TabBar, doesn't show. Is it suppose to disappear?
Navigation Items (Title, Left, Right and Back) of a Window are used only if the Window is directly presented by a NavigationBar. If you want to use a NavigationBar within your structure, you can add 3 NavigationBars inside the TabBar and put each Window in one of the NavigationBars.
From the "Best Practices" section of the NavigationBar documentation :
A common App schema is to have a root Window like a TabBar and a NavigationBar for each of its child. If the navigation is not in a single direction (from generic to detail and viceversa) then NavigationBar may not be appropriate.