CheckBox - How do I show my Three State checkbox to show the NULL state as a string value?

I want to use a checkbox that informs the user that the NULL value is "Unknown". I know the code to show that "Yes" if the checkbox is true & "No" if false but how would I code the NULL value in my table to show "Unknown"? Is this possible? I am using a Visual Basic form and SQL database. The datatype is set to a BIT.

Private Sub LicenceAgreeCheckBox_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles CheckBox.CheckedChanged
        If CheckBox.Checked = True Then
            CheckBox.Text = "Yes"
        Else
            CheckBox.Text = "No"
        End If
    End Sub

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