Gets or sets whether the button supports three states (checked, unchecked and indeterminate). This is a dependency property.
true if the button supports three states; false if the button supports two states (checked and unchecked).
A button supports three states (checked, unchecked and indeterminate) if its IsThreeState property is set to true. Otherwise, if this property is set to false, only checked and unchecked states are supported.
To set the button to the indeterminate state (if supported), set its SimpleButton.IsChecked property to null.