Sample code:

<ControlTemplate.Triggers>
<DataTrigger Value="True">
<DataTrigger.Binding>
<MultiBinding Converter="{StaticResource TestConverterConverter}" Mode="OneWay" >
<MultiBinding.Bindings>
<Binding ElementName="comboboxButton"/>
<Binding ElementName="comboboxButton" Path="IsChecked"/>
<Binding ElementName="popupContent" Path="ActualHeight"/>
</MultiBinding.Bindings>
</MultiBinding>
</DataTrigger.Binding>
<DataTrigger.Setters>
<Setter TargetName="SmoothComboBoxBorder" Property="BorderThickness">
<Setter.Value>
<MultiBinding Converter="{StaticResource ComboboxThicknessConverter}">
<Binding RelativeSource="{RelativeSource TemplatedParent}" Path="BorderThickness.Left"/>
<Binding Source="{StaticResource DefaultThickness}"/>
<Binding RelativeSource="{RelativeSource TemplatedParent}" Path="BorderThickness.Right"/>
<Binding RelativeSource="{RelativeSource TemplatedParent}" Path="BorderThickness.Bottom"/>
</MultiBinding>
</Setter.Value>
</Setter>
</DataTrigger.Setters>
</DataTrigger>

public sealed class TestConverterConverter : IMultiValueConverter
{
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
return true;
}

  public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
  {
     return new object[0];
  }

}