Avalonia/samples/IntegrationTestApp/Pages/RadioButtonPage.axaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
             x:Class="IntegrationTestApp.Pages.RadioButtonPage">
  <StackPanel Orientation="Vertical">
    <RadioButton Name="BasicRadioButton">Sample RadioButton</RadioButton>
    <StackPanel Orientation="Vertical">
      <RadioButton Name="ThreeStatesRadioButton1" IsChecked="True" IsThreeState="True">Three States: Option 1</RadioButton>
      <RadioButton Name="ThreeStatesRadioButton2" IsChecked="False" IsThreeState="True">Three States: Option 2</RadioButton>
    </StackPanel>
  </StackPanel>
</UserControl>