GD.ColumnDefinitions.Clear();
GD.ColumnDefinitions.Add(new ColumnDefinition());
GD.ColumnDefinitions.Add(new ColumnDefinition());
GD.Children.Add(***);
***.SetValue(Grid.ColumnProperty, 2);
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="3*"/>
<RowDefinition Height="2*"/>
</Grid.RowDefinitions>
<Grid Grid.Row="0">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="200" />
<ColumnDefinition />
<ColumnDefinition Width="200" />
</Grid.ColumnDefinitions>
<Button Content="btn1" Grid.Column="0" HorizontalAlignment="Left" Margin="48,60,0,0" VerticalAlignment="Top" Width="75"/>
<Button Content="btn2" Grid.Column="2" HorizontalAlignment="Left" Margin="60,60,0,0" VerticalAlignment="Top" Width="75"/>
</Grid>
<Grid Grid.Row="1">
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Border Grid.Row="0" Grid.Column="1">
<Button Content="btn3" />
</Border>
</Grid>
</Grid>