<Grid xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" IsSharedSizeScope="True">
  <Grid.ColumnDefinitions>
    <ColumnDefinition Width="Auto" SharedSizeGroup="myGroup"/>
    <ColumnDefinition/>
    <ColumnDefinition SharedSizeGroup="myGroup"/>
  </Grid.ColumnDefinitions>
  <Label Grid.Column="0" Background="Red"
  HorizontalContentAlignment="Center" VerticalContentAlignment="Center">
    1
  </Label>
  <GridSplitter Grid.Column="0" Width="5"/>
  <Label Grid.Column="1" Background="Orange"
  HorizontalContentAlignment="Center" VerticalContentAlignment="Center">
    2
  </Label>
  <Label Grid.Column="2" Background="Yellow"
  HorizontalContentAlignment="Center" VerticalContentAlignment="Center">
    3
  </Label>
</Grid>