How to delete all dependent rows

If you're not allowed to modify the tables, you can take a look at the EXISTS operator.
It will allow you to delete rows from a table only if the query inside the EXISTS return at least 1 result. You can use it to check dependencies.

You can write 3 queries:

DELETE C c
WHERE EXISTS (SELECT 1
              FROM B b
              WHERE c.b = b.b
              AND EXISTS (SELECT 1
                          FROM A a
                          WHERE a.a = b.a
                          AND ... ));
DELETE B b
WHERE EXISTS (SELECT 1
              FROM A a
              WHERE a.a = b.a
              AND ...);

DELETE A a
WHERE ...

The first one will take care of record in C that references records in B that references records in A that you want to delete.
Then you can remove records from B since there is no more dependencies in C.
Finally, you're able to delete records from A using the same logic.

You Can Use Cascade Cascading Referential Integrity Constraints

Update: you should enable Cascading Referential Integrity Constraints of Table A (PK) from Table B where ID of A is foreign Key and similarly of PK of Table B From Table C Where ID of B is foreign Key

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