When you model your building in RISAFloor, make sure that you define your slab edge as Flexible. That will ensure that the entire contents of the diaphragm will use flexible behavior.
You must define each Diaphragm Region in which flexible behavior occurs. In this structure there are clearly two regions which each act as a horizontal simply supported beam. The perimeter of each region must consist of lateral members to ensure a complete load path. The beams on Grids A and C must be lateral so that they can act as collectors for the walls.
After bringing the model to RISA-3D it is obvious by viewing the loads or the deflected shape that the wall along Grid 3 is taking half of the overall seismic load (as it should with flexible behavior) as opposed to one third in rigid behavior. The diaphragm panel and nailing are also reported.