Comment by skyde

Curious why your primary key are GUID? Also if the primary key of table C is made of 3 column (A GUID, B GUID, C GUID) the index in PostgreSQL will not be that big with compression enabled because all the common prefix will not be stored redundantly.

Also having a single compound index on table C covering column (A ForeingKey, B ForeingKey, C GUID) is much better than having multiple index on table C.