OK, what is the recordsource of your form? the field list is based on that. That's why I said that you will need to modify your form's recordsource.
You can create a query which contains all the fields in all the tables, no matter if you add fields, by using the asterisk from the top of the field list in query design.
If you have three tables that are the basis of your form, create a query, add all three tables and drag the links between them, if you haven't already set up relationships.
then for each field, doubleclick on the asterisk which is listed at the top of the field list of each table. That tells the query to get all the fields. Make this query the recordsource of your form.
If you add fields, they will automatically be added to the query, and therefore will be part of your recordsource, and therefore will be available in the field list.
I hope that I understood your question.
Kathryn