How to add an additional table over two one to many relations tables?
If the relationship between TableA and TableB is meant to be one-to-many then you don't need a third table (TableC).
However, if TableA to TableB are meant to have a many-to-many relationship, then you would need a third table (TableC).  In this case you would have foreign keys in TableC to both TableA and TableB.  Normally these two foreign keys in TableC would have a unique constraint (and probably index) applied to the combination of these two keys (a compound key).
Often this compound key is also used as the primary key of TableC.  This doesn't have to be the case. It's possible for TableC to have it's own single column unique key.  It depends on the nature of the data you're storing in TableC.