If you want quick and direct access to the record, you could do it like that, yes.
I have to say that personally, I always try to avoid compound keys, and go for a single auto-increment 'id' column als primary key. So I don't have a lot of experience with it.
If Model_Something has a compound primary key with 3 columns, how many are foreign keys? In the forge() array, you should only pass the data for columns that are not foreign keys.
Also, make sure that your relationship is defined correctly, so if Model_Something has two foreign keys linking it to Model_A, they should both be defined in the has_many and belongs_to definition.