Sample three-state CheckBox using Paths

AlertCheckBox.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <StreamGeometry x:Key="Blank"></StreamGeometry>
    <StreamGeometry x:Key="CheckBoxUnchecked">M5.75,3 L18.25,3 C19.7687831,3 21,4.23121694 21,5.75 L21,18.25 C21,19.7687831 19.7687831,21 18.25,21 L5.75,21 C4.23121694,21 3,19.7687831 3,18.25 L3,5.75 C3,4.23121694 4.23121694,3 5.75,3 Z M5.75,4.5 C5.05964406,4.5 4.5,5.05964406 4.5,5.75 L4.5,18.25 C4.5,18.9403559 5.05964406,19.5 5.75,19.5 L18.25,19.5 C18.9403559,19.5 19.5,18.9403559 19.5,18.25 L19.5,5.75 C19.5,5.05964406 18.9403559,4.5 18.25,4.5 L5.75,4.5 Z</StreamGeometry>
    <StreamGeometry x:Key="CheckBoxChecked">M18.25,3 C19.7687831,3 21,4.23121694 21,5.75 L21,18.25 C21,19.7687831 19.7687831,21 18.25,21 L5.75,21 C4.23121694,21 3,19.7687831 3,18.25 L3,5.75 C3,4.23121694 4.23121694,3 5.75,3 L18.25,3 Z M18.25,4.5 L5.75,4.5 C5.05964406,4.5 4.5,5.05964406 4.5,5.75 L4.5,18.25 C4.5,18.9403559 5.05964406,19.5 5.75,19.5 L18.25,19.5 C18.9403559,19.5 19.5,18.9403559 19.5,18.25 L19.5,5.75 C19.5,5.05964406 18.9403559,4.5 18.25,4.5 Z M10,14.4393398 L16.4696699,7.96966991 C16.7625631,7.6767767 17.2374369,7.6767767 17.5303301,7.96966991 C17.7965966,8.23593648 17.8208027,8.65260016 17.6029482,8.94621165 L17.5303301,9.03033009 L10.5303301,16.0303301 C10.2640635,16.2965966 9.84739984,16.3208027 9.55378835,16.1029482 L9.46966991,16.0303301 L6.46966991,13.0303301 C6.1767767,12.7374369 6.1767767,12.2625631 6.46966991,11.9696699 C6.73593648,11.7034034 7.15260016,11.6791973 7.44621165,11.8970518 L7.53033009,11.9696699 L10,14.4393398 L16.4696699,7.96966991 L10,14.4393398 Z</StreamGeometry>
    <StreamGeometry x:Key="WarningRegular">M10.9093922,2.78216375 C11.9491636,2.20625071 13.2471955,2.54089334 13.8850247,3.52240345 L13.9678229,3.66023048 L21.7267791,17.6684928 C21.9115773,18.0021332 22.0085303,18.3772743 22.0085303,18.7586748 C22.0085303,19.9495388 21.0833687,20.9243197 19.9125791,21.003484 L19.7585303,21.0086748 L4.24277801,21.0086748 C3.86146742,21.0086748 3.48641186,20.9117674 3.15282824,20.7270522 C2.11298886,20.1512618 1.7079483,18.8734454 2.20150311,17.8120352 L2.27440063,17.668725 L10.0311968,3.66046274 C10.2357246,3.291099 10.5400526,2.98673515 10.9093922,2.78216375 Z M20.4146132,18.3952808 L12.6556571,4.3870185 C12.4549601,4.02467391 11.9985248,3.89363262 11.6361802,4.09432959 C11.5438453,4.14547244 11.4637001,4.21532637 11.4006367,4.29899869 L11.3434484,4.38709592 L3.58665221,18.3953582 C3.385998,18.7577265 3.51709315,19.2141464 3.87946142,19.4148006 C3.96285732,19.4609794 4.05402922,19.4906942 4.14802472,19.5026655 L4.24277801,19.5086748 L19.7585303,19.5086748 C20.1727439,19.5086748 20.5085303,19.1728883 20.5085303,18.7586748 C20.5085303,18.6633247 20.4903516,18.5691482 20.455275,18.4811011 L20.4146132,18.3952808 L12.6556571,4.3870185 L20.4146132,18.3952808 Z M12.0004478,16.0017852 C12.5519939,16.0017852 12.9991104,16.4489016 12.9991104,17.0004478 C12.9991104,17.5519939 12.5519939,17.9991104 12.0004478,17.9991104 C11.4489016,17.9991104 11.0017852,17.5519939 11.0017852,17.0004478 C11.0017852,16.4489016 11.4489016,16.0017852 12.0004478,16.0017852 Z M11.9962476,8.49954934 C12.3759432,8.49924613 12.689964,8.78114897 12.7399193,9.14718469 L12.7468472,9.24894974 L12.750448,13.7505438 C12.7507788,14.1647572 12.4152611,14.5008121 12.0010476,14.5011439 C11.621352,14.5014471 11.3073312,14.2195442 11.257376,13.8535085 L11.250448,13.7517435 L11.2468472,9.25014944 C11.2465164,8.83593601 11.5820341,8.49988112 11.9962476,8.49954934 Z</StreamGeometry>
    <StreamGeometry x:Key="ForwardSlash">M 0 7 L 7 0</StreamGeometry>
    <StreamGeometry x:Key="CheckMark">M103.78572,598.96112 L105.09846,597.5661 L107.00806,600.16229 C107.00806,600.16229 109.91004,592.74463 109.91004,592.74463 C109.91004,592.74463 111.74678,593.79761 111.74678,593.79761 C111.74678,593.79761 107.88566,602.75848 107.88566,602.75848 L106.60118,602.75848 z</StreamGeometry>
    <StreamGeometry x:Key="CheckMarkRegular">M4.03033009,13.4696699 C3.73743687,13.1767767 3.26256313,13.1767767 2.96966991,13.4696699 C2.6767767,13.7625631 2.6767767,14.2374369 2.96966991,14.5303301 L9.96966991,21.5303301 C10.2625631,21.8232233 10.7374369,21.8232233 11.0303301,21.5303301 L25.0303301,7.53033009 C25.3232233,7.23743687 25.3232233,6.76256313 25.0303301,6.46966991 C24.7374369,6.1767767 24.2625631,6.1767767 23.9696699,6.46966991 L10.5,19.9393398 L4.03033009,13.4696699 Z</StreamGeometry>
    <Style x:Key="AlertCheckBox" TargetType="{x:Type CheckBox}">
        <Setter Property="IsHitTestVisible" Value="False" />
        <Setter Property="IsThreeState" Value="True" />
        <Setter Property="VerticalAlignment" Value="Center" />
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type CheckBox}">
                    <BulletDecorator Background="Transparent">
                        <BulletDecorator.Bullet>
                            <Grid>
                                <Path x:Name="CheckMark"
                                      SnapsToDevicePixels="False"
                                      Stretch="Uniform"
                                      Fill="White"
                                      Stroke="White"
                                      StrokeThickness="0"
                                      Data="" />
                            </Grid>
                        </BulletDecorator.Bullet>
                        <!--<ContentPresenter Margin="4,0,0,0"
                                          VerticalAlignment="Center"
                                          HorizontalAlignment="Left"
                                          RecognizesAccessKey="True" />-->
                    </BulletDecorator>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsChecked" Value="True">
                            <Setter TargetName="CheckMark"
                                    Property="Data"
                                    Value="{StaticResource CheckMark}" />
                        </Trigger>
                        <Trigger Property="IsChecked" Value="False">
                            <Setter TargetName="CheckMark"
                                    Property="Data"
                                    Value="{StaticResource Blank}" />
                        </Trigger>
                        <Trigger Property="IsChecked" Value="{x:Null}">
                            <Setter TargetName="CheckMark"
                                    Property="Data"
                                    Value="{StaticResource WarningRegular}" />
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>

SampleView.xaml

<CheckBox Style="{StaticResource AlertCheckBox}" />