<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="35"/>
        <RowDefinition/>
    </Grid.RowDefinitions>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="150"/>
        <ColumnDefinition Width="Auto"/>
        <ColumnDefinition/>
    </Grid.ColumnDefinitions>

    <TextBox Grid.ColumnSpan="3" BorderBrush="Black"/>
    <TextBox Grid.Row="1" BorderBrush="Black"/>
    <GridSplitter Grid.Row="1" Grid.Column="1" VerticalAlignment="Stretch" HorizontalAlignment="Center" Width="3" Background="Gray" ShowsPreview="True"/>
    <TextBox Grid.Row="1" Grid.Column="2" BorderBrush="Black"/>
</Grid>

<GridSplitter/> 放在 <ColumnDefinition Width="Auto"/>里面自适应宽度。

参考：
《WPF深入浅出》 P73

WPF 可拖拽的分隔栏 GridSpliter