WPF storyboard for the three-state checkbox

I have a WPF application and three-state CheckBox on it. I created a custom template for the check-box and want animate it like a slider (move slider left and right). I've created a slider and set the triggers:

<ControlTemplate TargetType="{x:Type CheckBox}">
    <ControlTemplate.Triggers>
        <Trigger Property="IsChecked" Value="True">
            <Trigger.EnterActions>
                <BeginStoryboard Storyboard="{StaticResource SwitchOn}" />
            </Trigger.EnterActions>
            <Trigger.ExitActions>
                <BeginStoryboard Storyboard="{StaticResource SwitchOff}" />
            </Trigger.ExitActions>
        </Trigger>
        <Trigger Property="IsChecked" Value="False">
            <Trigger.EnterActions>
                <BeginStoryboard Storyboard="{StaticResource SwitchOff}" />
            </Trigger.EnterActions>
            <Trigger.ExitActions>
                <BeginStoryboard Storyboard="{StaticResource SwitchOn}" />
            </Trigger.ExitActions>
        </Trigger>
        <Trigger Property="IsChecked" Value="{x:Null}">
            <Trigger.EnterActions>
                <BeginStoryboard Storyboard="{StaticResource SwitchMiddle}" />
            </Trigger.EnterActions>
            <Trigger.ExitActions>
                <BeginStoryboard Storyboard="{StaticResource SwitchOff}" />
            </Trigger.ExitActions>
        </Trigger>
    </ControlTemplate.Triggers>
</ControlTemplate>

This style applied to the two-state checkbox is works perfectly on checkbox toggle in such cycle: switch on -> switch off ... and so on.

But when I apply it to three-state checkbox, the animation cycle is works good (what I need) only once: switch on -> switch middle -> switch off

And after that the cycle is repeats like this: switch off -> switch middle -> switch off -> switch off -> switch middle ...

I tried many different combinations of the enter actions and exit actions, but unsuccessfully. What is the right way to create a triggers to excpected behavior? Thanks!