Newbie Link Tables Problem

I'm building a simple db to track people with access to systems and buildings (Access 2000). I have three tables:

1. People (names, ID, etc.) with a primary key of an autonumber
2. Bldgs - y/n fields of bldg #'s with a primary key of a number (linked one to many from the People table)
3. Systems - essentially the same set up as Bldgs table (though they're not related)

People may have access to either bldgs, systems, both or none.

So I build a query to combine all three tables. If I add a new record (to the People table and only one of the other two tables) I can't go back later and add data to the third linked table seemingly because the autonumber exists in the other two tables but not that one. Well, sure.

So the question is, I'm obviously missing something insanely obvious, or if not, how do I get around this? It's driving me nuts.

Thanks,