How to set/reset Three-state checkbox value in WPF

I have a datagrid whose one of the header column is Three-state checkbox. The celltemplate for that column contains two state checkbox + AllItems CheckBox - Item1 - Item2 - Item3 .. I wanted to use AllItems checkbox to select/unselect all items (item1,item2) which works fine. Next I wanted to set AllItems checkbox to intermediate state when not all items are selected/unselected. Similarly I wanted to set AllItems checkbox as checked/unchecked when all items get manually selected.

Here is the code that I tried...

<dg:DataGridTemplateColumn.HeaderTemplate>
    <DataTemplate>
        <StackPanel x:Name="StackPanelForItemCheckbox" Orientation="Horizontal">
           <CheckBox x:Name="AllItemSelectionCheckBox" HorizontalAlignment="Left" Cursor="Hand"  
                     IsChecked="{Binding IsAllItemChecked, Mode=TwoWay}"                                              
                     IsThreeState="True"  Checked="ItemSelectionCheckBox_Checked" 
                     Unchecked="ItemSelectionCheckBox_Unchecked"
                     Click="AllItemSelectionCheckBox_Click">
           <TextBlock x:Name="ItemNameTextBlock" Text="Item" Margin="10,0,0,0">
           ......
<dg:DataGridTemplateColumn.CellTemplate>
       <DataTemplate x:Name="ItemCheckDataTemplate">                                
           <StackPanel x:Name="ItemCheckBoxStackPanel" Orientation="Horizontal">                                    
                  <CheckBox x:Name="itemCheckBox" Cursor="Hand" IsChecked="{Binding IsItemChecked, Mode=TwoWay}" Click="ItemSelectionCheckBox_Click"></CheckBox>
                   <TextBlock x:Name="ItemNameTextBlock" Text="{Binding Path=Item}"> </TextBlock>                                   
            </StackPanel>
         </DataTemplate>
...

"ItemSelectionCheckBox_Click" method looks for all three state (all-checked, none-ch