I'm wondering if this can be done in the Advanced Data Grid :
I've got three MySQL joined tables with one to many relationships happening. (#1 can have multiple #2, and each #2 can have multiple #3).
I'm looking to build a simple CRUD (create, read, update, delete).
Can flex write back the data to the joined tables?
Take a look at this example in how to update your database using mysql:
You can update join tables, you have to do one at the time using your primary keys in the where clause.