Top Dock in a DockPanel is taking all the space

My code looks something like this:

<DockPanel>
  <Expander DockPanel.Dock="Top" Name="Expander1">
     <local:ListView1 DataContext="{Binding Source1}"/>
  </Expander>
  <Expander DockPanel.Dock="Top" Name="Expander2">
     <local:ListView1 DataContext="{Binding Source2}"/>
  </Expander>
  <Expander DockPanel.Dock="Top" Name="Expander3">
     <local:ListView1 DataContext="{Binding Source3}"/>
  </Expander>
</DockPanel>

ListView1 is just a user control that contains a ListView

I have the behavior set: when one Expander is open, all the other 2 Expanders will close.

The problem is that when Expander1 is open and its content is more than the window height, it will have a scroll bar to scroll down for its content while Expander2 and Expander3 are not displayed. I think Expander1 uses all the space on the UI and Expander2 & Expander3 get pushed out side of the UI. When Expander2 is open, Expander3 is pushed out of the UI and not displayed. What can I do so that when I open an Expander, the one(s) below it won't get pushed out of the UI?

You said you're only allowing one open at a time.

Given that then the size logic is simpler and you could use just a converter with a fixed number.

My PoC:

<DockPanel>
    <Expander DockPanel.Dock="Top" MaxHeight="{Binding ActualHeight, 
        RelativeSource={RelativeSource AncestorType=ContentPresenter}, 
        Converter={local:AddConverter ValueToAdd=-46}}">
        <ListBox ItemsSource="{Binding Items}"/>
    </Expander>
    <Expander DockPanel.Dock="Top" MaxHeight="{Binding ActualHeight, 
        RelativeSource={RelativeSource AncestorType=ContentPresenter}, 
        Converter={local:AddConverter ValueToAdd=-46}}">
        <ListBox ItemsSource="{Binding Items}"/>
    </Expander>
    <Expander DockPanel.Dock="Top" MaxHeight="{Binding ActualHeight, 
        RelativeSource={RelativeSource AncestorType=ContentPresenter}, 
        Converter={local:AddConverter ValueToAdd=-46}}">
        <ListBox ItemsSource="{Binding Items}"/>
    </Expander>
</DockPanel>

And the converter

public class AddConverter : MarkupExtension, IValueConverter
{
    public double ValueToAdd { get; set; }

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        double bound = (Double)value;
        return bound + ValueToAdd;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return null;
    }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        return this;
    }
}

As I mentioned in the comments.

You could make this more sophisticated with a multibinding and multiconverter. That could allow for height when 2 are open.