each of 3 Masters to 1 Detail in Oracle Forms: how to arrange?

I have up to 3 instances of a Master (permit) showing on the form; for each master there can be only one Detail (vehicle). There are with the restrictions I have set (ID, expiry year of permit) two instances in the database; the Master appears for both but the Detail only for the first. Presumably I should change something in the data block property palettes, but I am not sure what. I am adapting from an earlier part of the form, where only one Master is allowed, with up to 3 Details, and that works.