如何在wpf treeview上对齐按钮

<TreeView>
    <TreeViewItem Header="Parent Item">
        <TreeViewItem Header="Child Item 1">
            <TreeViewItem.HeaderTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <Button Content="Left Aligned" HorizontalAlignment="Left" />
                        <Button Content="Right Aligned" HorizontalAlignment="Right" />
                        <Button Content="Center Aligned" HorizontalAlignment="Center" />
                    </StackPanel>
                </DataTemplate>
            </TreeViewItem.HeaderTemplate>
        </TreeViewItem>
        <TreeViewItem Header="Child Item 2">
            <TreeViewItem.HeaderTemplate>
                <DataTemplate>
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="*" />
                            <ColumnDefinition Width="*" />
                            <ColumnDefinition Width="*" />
                        </Grid.ColumnDefinitions>
                        <Button Content="Left Aligned" HorizontalAlignment="Left" />
                        <Button Content="Right Aligned" HorizontalAlignment="Right" Grid.Column="1" />
                        <Button Content="Center Aligned" HorizontalAlignment="Center" Grid.Column="2" />
                    </Grid>
                </DataTemplate>
            </TreeViewItem.HeaderTemplate>
        </TreeViewItem>
    </TreeViewItem>
</TreeView>