complextree.xaml

        <TreeView x:Name="questionTree" FontSize="18"
        HorizontalAlignment="Left"  Width="280" Margin="10,48,0,35" ItemsSource="{Binding RootGroups}" SelectedItemChanged="questionTree_SelectedItemChanged" >
            <TreeView.Resources>
                <HierarchicalDataTemplate DataType="{x:Type qa:QAQuestionGroup}" ItemsSource="{Binding SubGroups}">
                    <TextBlock Text="{Binding}" />
                    
                    <HierarchicalDataTemplate.ItemTemplate>

                        <HierarchicalDataTemplate DataType="{x:Type qa:QAQuestion}" ItemsSource="{Binding Questions}">
                            <HierarchicalDataTemplate.ItemTemplate>
                                <DataTemplate>
                                    <TextBlock Text="{Binding QuestionName}" />
                                </DataTemplate>
                            </HierarchicalDataTemplate.ItemTemplate>
                        </HierarchicalDataTemplate>
                    </HierarchicalDataTemplate.ItemTemplate>
                </HierarchicalDataTemplate>

            </TreeView.Resources>
        </TreeView>

public class QAQuestionGroup
{
	public string GroupName {get;set;}
	public virtual ICollection<QAQuestion> Questions {get;set;}
	public virtual ICollection<QAQuestionGroup> SubGroups {get;set;}
}

public class QAQuestion
{
	public string QuestionName {get;set;}
}