CheckBox controls
The ThirdState property specifies whether the CheckBox is in the third state (neither selected nor unselected). For a check box to be in the third state, the ThreeState property must also be enabled.
To specify that a check box is in the third state:
Check both the ThreeState and the ThirdState check boxes on the General page of the control’s Properties view.
The ThirdState property takes a boolean value. The following lines specify that a CheckBox can have three states and that it starts out in the third state:
cbx_1.ThreeState = TRUE
cbx_1.ThirdState = TRUE
