Set column span by using Grid.Children.Add from source code

I have the following source code for setting up a Xamarin.Forms.Grid control on my page. I will like the first 3 rows to expand both columns. The code sample in the online documentation does not really explain how this works.

Here is the source code:

Grid myGrid = new Grid();
myGrid.HorizontalOptions = LayoutOptions.FillAndExpand;

myGrid.RowDefinitions = new RowDefinitionCollection
{
        new RowDefinition { Height = new GridLength(50, GridUnitType.Absolute) },
                new RowDefinition { Height = GridLength.Auto },
                new RowDefinition { Height = GridLength.Auto },
                new RowDefinition { Height = new GridLength(30, GridUnitType.Absolute) }
};

myGrid.ColumnDefinitions = new ColumnDefinitionCollection
{
        new ColumnDefinition { Width = GridLength.Auto },
        new ColumnDefinition { Width = new GridLength(30, GridUnitType.Absolute) }
};

//I need Rows 1,2,3 to span two columns
myGrid.Children.Add(new Label { Text = "Row 1" }, 0, 0);
myGrid.Children.Add(new Label { Text = "Row 2" }, 0, 1);
myGrid.Children.Add(new Label { Text = "Row 3" }, 0, 2);
myGrid.Children.Add(new Label { Text = "Row 4, column 1" }, 0, 3);
myGrid.Children.Add(new Label { Text = "Row 4, column 2" }, 1, 3);

How do I modify the code to have the first 3 rows span 2 columns?