Module Relationships and Question about counting number entries in multiple fields.
Using Jamroom
Yes - I would recommend checking out the User Comments module (jrComment) - it does this. Basically it stored a "comment_module" and "comment_item_id" key for each comment that is made that contains the module and item that the comment was made for.
As for doing math on some fields and storing it to a 4th field, no - the form designer is not going to help with that. Instead, in your custom module what you want to do is simply compute it on access via "db_get_item" and "db_search_items" event listeners - again check out the following function:
jrComment_db_get_item_listener()
in the jrComment/include.php file to see how it "listens" for the DataStore access on a comment and adds in the info about the item. You would want to run your computation on your fields and add in the key for the 4th field right in the listener.
Hope this helps!