At any one time only one of the fields should be checked.
Why not use one field, with three choices in a radio button set?
That would be fine but the check boxes are on different tabs.
It's ok guys I've found a way around the problem.
the check boxes are on different tabs.
You can still use a single fied, duplicated among the tabs. If formatted as a radio button, each succesive choice will replace the previous one - no matter in which instance of the field it is made.
To protect against user holding Shift while selecting, set the field to auto-enter:
GetValue ( Self ; ValueCount ( Self ) )