使用UniformGrid可以实现所需的行为。
确保将行数设置为1。

<UniformGrid Rows="1">
    <Label>One</Label>
    <Label>Two</Label>
    <Label>Three</Label>
</UniformGrid>

其余元素的间距相等。

<UniformGrid Rows="1">
    <Label>One</Label>
    <Label Visibility="Collapsed">Two</Label>
    <Label>Three</Label>
</UniformGrid>

我按照Rob的建议添加了Xaml绑定，如下所示：

<Grid.ColumnDefinitions>
  <ColumnDefinition Width="*" />
  <ColumnDefinition Width="{Binding MiddleColumnWidth}" />
  <ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>

<Label Grid.Row="0" Grid.Column="0">One</Label>
<Label Grid.Row="0" Grid.Column="1" Visibility="{Binding IsSecondLabelVisible}">Two</Label>
<Label Grid.Row="0" Grid.Column="2">Three</Label>

xaml背后的代码：

private bool ShowOnlyTwoColumns;

private GridLength MiddleColumnWidth
{
  get
  {
    if (ShowOnlyTwoColumns)
      return GridLength.Auto; // Auto collapses the grid column when label is collapsed

    return new GridLength(1, GridUnitType.Star);
  }
}

private Visibility IsSecondLabelVisible
{
  get { return this.ShowOnlyTwoColumns ? Visibility.Collapsed : Visibility.Visible; }
}

我迟到了，但我想提出我的解决方案，以防它对任何人有用。
我用这个转换器

public class BooleanToAutoOrStarGridLengthConverter : MarkupExtension, IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value is bool isStar)
            return new GridLength(1, isStar ? GridUnitType.Star : GridUnitType.Auto);
        return new GridLength(0, GridUnitType.Star);
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value is GridLength gridLength && gridLength.GridUnitType == GridUnitType.Star)
            return true;
        return false;
    }

    public override object ProvideValue(IServiceProvider serviceProvider) => this;
}

如下所示：

<Grid.ColumnDefinitions>
  <ColumnDefinition Width="*" />
  <ColumnDefinition Width="{Binding IsSecondLabelVisible, Converter={BooleanToAutoOrStarGridLengthConverter}}" />
  <ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>

<Label Grid.Row="0" Grid.Column="0">One</Label>
<Label Grid.Row="0" Grid.Column="1" Visibility="{Binding IsSecondLabelVisible}">Two</Label>
<Label Grid.Row="0" Grid.Column="2">Three</Label>