I have a file with three related tables, A, B and C. The relationships between A and B, and B and C, are predicated on different fields, but for each new record in C, there's only one possible related record each from A and B.

My layout for C contains fields from both A and B. When I create a new record in C, the B fields are always filled instantly, but the A fields frequently don't come in even when the record is committed. They always show up in the long run, but it sometimes requires closing and reopening the file. I've also found that if I start filling in one of the A fields in my C layout, it prompts the rest of the A fields to fill themselves. Is there a more streamlined way of doing this?

Sure: add a direct relationship from A to C.