Hi, I've 3 tables, A that is sure the master one, B that is detail
respect to A, and C that is a detail for the A and B current row,
i.e.
the A+B primary key are the foreign key for the C detail table.
Since the TIB_Query has only one possible reference for a master
datasource, what is the best way, eventually better code location,
for
solving this problem?

Think something like:
A customer table
B logical places (i.e. home, office, parents, mobile)
C telephone numbers
So Mary at home has 3 rows of telephone numbers
Mary at office has 2 numbers
Mary has one mobile phone number

C (mary) B C
|----------- home -------800 2554444
| |----------750 0045444
| |----------544 4445454
|
|----------- office -----450 5554444
| |----------754 0455554
|
|---------- mobile ------477 5554444

So in the first IB_Grid I select Mary, then in the second I select
office, and the 3° shows me her office telephone numbers...

Thanks

Marco Menardi