Deleting "child" records on delete (reverse foreign key constraint?) To illustrate my problem, I have 3 tables.

Table A AND Table B both individually have FK C_ID that reference Table C's PK. 

I want Table C's rows to be deleted if their related rows in Table A or Table B is deleted.

FK constraints seem to only work one way. How do I deal with this?

To clarify, A and B has a 1:1 relationship with C. To be more precise it's a (A,B)parent-child(C) relationship where the child's existence depends on the parent's.
I would use a delete trigger to remove the records from table C when a delete occurs on table a or b.

There is a question from SO where the answer gives you a nice example: 

https://stackoverflow.com/questions/11818191/mysql-trigger-delete-from-table-after-delete

But to give you the rough idea from that answer:

CREATE TRIGGER table_c_delete AFTER DELETE on table_a
FOR EACH ROW
BEGIN
DELETE FROM table_c
    WHERE table_c.parent_id = old.id;
END