If you put the primary key of table 1 also in table 3, your associations become less useful, perhaps useless. Fortunately 'has_many :through' gives you what you want without messing up your associations.
ModelA has_many :model_bs has_many :modelcs, :through :model_b
ModelB belongs_to :model_a has_many :model_cs
ModelC belongs_to :model_b
This way you can refer to ModelA.model_cs directly, and Rails will do the work of putting the right information into the SQL call.
--f