XAML
IsThreeState="True" - третій стан
<StackPanel x:Name="stackPanel">
<CheckBox x:Name="checkBox1" IsThreeState="True" IsChecked="False" Height="20" Content="Сніданок" />
<CheckBox x:Name="checkBox2" IsThreeState="True" IsChecked="True" Height="20" Content="Обід" />
<CheckBox x:Name="checkBox3" IsThreeState="True" IsChecked="{x:Null}" Height="20" Content="Вечеря"/>
</StackPanel>
C#
if(checkBox1.IsChecked == true) {
}