This has been covered here and a zillion other places and is known as cascading combos. Rather than repeat what's already been covered to death, suggest you Google it using that term. If you get stuck, post back with more information, including what the combos are based on (query/table, field list or value list) and be sure to post any sql or other related info about what you tried and what the results were.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.