couple of things that might affect the rating, agreed with Robert that we need to know if 9.5 of the OFC applies, but further to that, does the attic contain any building services? does the other unit have access to the attic? is it being used for storage space? does it need detection?
most tri-plexes i come across i have found that they have been converted from a single family dwelling at one time without a permit, you may want to considering asking the building dept if there is any history on this building.