Traté de resolver mi anteriorpregunta con unir manualmente elWidth propiedad de laDataGridTextColumn Aquí está la primera versión de mi código XAML.

   <DataGrid AutoGenerateColumns="False" Background="White" ItemsSource="{Binding Items, Mode=OneWay}" 
              HorizontalGridLinesBrush="Silver" VerticalGridLinesBrush="Silver"
              Margin="332,10,10,10" CanUserAddRows="False" CanUserDeleteRows="False"
              x:Name="myDataGrid" ColumnWidth="*">
        <DataGrid.Columns>
            <DataGridTextColumn Width="{Binding Path=ActualWidth, Converter={StaticResource ResourceKey=WidthValueConverter}, RelativeSource={RelativeSource AncestorType=DataGrid}}" IsReadOnly="True" Header="Column1" Binding="{Binding Value1, Mode=OneWay}" />
            <DataGridTextColumn Width="{Binding Path=ActualWidth, Converter={StaticResource ResourceKey=WidthValueConverter}, RelativeSource={RelativeSource AncestorType=DataGrid}}" IsReadOnly="True" Header="Column2" Binding="{Binding Value2, Mode=OneWay}"/>
            <DataGridTextColumn Width="{Binding Path=ActualWidth, Converter={StaticResource ResourceKey=WidthValueConverter}, RelativeSource={RelativeSource AncestorType=DataGrid}}" IsReadOnly="True" Header="Column3" Binding="{Binding Value3, Mode=OneWay}"/>
        </DataGrid.Columns>
    </DataGrid>

Después de una pequeña investigación encontré estoenviar Eso parece proporcionar la respuesta a mi problema y actualicé miDataGrid código.

   <DataGrid AutoGenerateColumns="False" Background="White" ItemsSource="{Binding Items, Mode=OneWay}" 
              HorizontalGridLinesBrush="Silver" VerticalGridLinesBrush="Silver"
              Margin="332,10,10,10" CanUserAddRows="False" CanUserDeleteRows="False"
              x:Name="myDataGrid" ColumnWidth="*">
        <DataGrid.Columns>
            <DataGridTextColumn Width="{Binding Path=ActualWidth, Converter={StaticResource ResourceKey=WidthValueConverter}, Source={x:Reference Name=myDataGrid}}" IsReadOnly="True" Header="Column1" Binding="{Binding Value1, Mode=OneWay}" />
            <DataGridTextColumn Width="{Binding Path=ActualWidth, Converter={StaticResource ResourceKey=WidthValueConverter}, Source={x:Reference Name=myDataGrid}}" IsReadOnly="True" Header="Column2" Binding="{Binding Value2, Mode=OneWay}"/>
            <DataGridTextColumn Width="{Binding Path=ActualWidth, Converter={StaticResource ResourceKey=WidthValueConverter}, Source={x:Reference Name=myDataGrid}}" IsReadOnly="True" Header="Column3" Binding="{Binding Value3, Mode=OneWay}"/>
        </DataGrid.Columns>
    </DataGrid>

pero ahora estoy recibiendo esta XamlParseException

Cannot call MarkupExtension.ProvideValue because of a cyclical dependency. Properties inside a 
MarkupExtension cannot reference objects that reference the result of the MarkupExtension. 
The affected MarkupExtensions are:
'System.Windows.Data.Binding' Line number '37' and line position '37'.
'System.Windows.Data.Binding' Line number '38' and line position '37'.
'System.Windows.Data.Binding' Line number '39' and line position '37'.

Entonces, ¿cómo puedo atar elWidth propiedad de unDataGridColumn alActualWidth propiedad de su padreDataGrid