A family of Microsoft word processing software products for creating web, email, and print documents.
What type of checkboxes have you used for the form? - Content Control, Legacy Forms, or Active X
To avoid the issue where more that one of the check boxes are ticked, you might want to consider using a combo-box control that contains the items:
Needs Improvement
Successful
Exceeds Expectations
However, whatever way that you do it, you are going to need to use some VBA code to produce the desired result and the code will depend upon the type of controls that are being used.
If you send me a copy of the form, referencing this thread in the covering email message, I will take a look at the issue.