How to group fields when creating a fillable PDF?

Hello,

I have a form in word that contains 3 check boxes in a row in sebsequent columns.  When I convert it to PDF, select Tools, select Create (from exisitng document, current document) it auto populates the fields it can recognize.  Luckily it recognizes the check boxes that I already have.  The problem is that the form recognizes one of the rows of check boxes as a group of check boxes but a different row of check boxes are not grouped together.  The grouped check boxes make it so that only one check box can be selected at a time.  The ungrouped check boxes gives the user the ability to chek multiple of the boxes.  When I am looking at the fields pain on the right hand side I can see the difference between the grouped check boxes and the check boxes without a group.  I want to know how to group specific fields together manually when converting it from word to PDF didn't automatically group them together.

Thank you,

Aaron