How do I add a back, forward, share buttons in the nav bottom bar?

Nav bottom bar has inbuilt items as given below:

{
    Done,
    Cancel,
    Edit,
    Save,
    Add,
    FlexibleSpace,
    FixedSpace,
    Compose,
    Reply,
    Action,
    Organize,
    Bookmarks,
    Search,
    Refresh,
    Stop,
    Camera,
    Trash,
    Play,
    Pause,
    Rewind,
    FastForward,
    Undo,
    Redo,
    [Introduced (PlatformName.iOS, 4, 0, PlatformArchitecture.None, null)]
    PageCurl
}

}

But I am looking for front, back, share. How do I get them?