How to save single value from a select field?
I would use jQuery to deactivate and uncheck the other two checkboxes when the third has been checked. If they uncheck the third, they will all become active.

Your Drupal form will look something like this:

&lt;div class="form-checkboxes" id="edit-field-terms-und"&gt;
&lt;div class="form-item form-type-checkbox form-item-field-terms-und-pr"&gt;
    &lt;input type="checkbox" class="form-checkbox" value="pr" name="field_terms[und][pr]" id="edit-field-terms-und-pr"&gt;  &lt;label for="edit-field-terms-und-pr" class="option"&gt;Print Required&lt;/label&gt;
&lt;/div&gt;
&lt;div class="form-item form-type-checkbox form-item-field-terms-und-dp"&gt;
    &lt;input type="checkbox" class="form-checkbox" value="dp" name="field_terms[und][dp]" id="edit-field-terms-und-dp"&gt;  &lt;label for="edit-field-terms-und-dp" class="option"&gt;Digital Print Required&lt;/label&gt;
&lt;/div&gt;
&lt;div class="form-item form-type-checkbox form-item-field-terms-und-tp"&gt;
    &lt;input type="checkbox" class="form-checkbox" value="tp" name="field_terms[und][tp]" id="edit-field-terms-und-tp"&gt;  &lt;label for="edit-field-terms-und-tp" class="option"&gt;Print has been taken&lt;/label&gt;
&lt;/div&gt;
&lt;/div&gt;


jQuery code would be something like this:

(function($) {
$(document).ready(function() { 

    $("#edit-field-terms-und-tp").click(function() {
        if($(this).is(':checked')) { 
            $('#edit-field-terms-und-pr, #edit-field-terms-und-dp').attr('disabled', true).prop('checked', false );
        } else {
            $('#edit-field-terms-und-pr, #edit-field-terms-und-dp').removeAttr('disabled');
        }
    });

});
})(jQuery);


Haven't tested it, but it should work.