Grid.SharedSizeGroup

 <Grid 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>