Will I need physical therapy for three posterior rib fractures?

Not typically. In general, therapy in a patient with healing fractures of the ribs can be painful and not particularly helpful; other than in the initial mobilization to get a person out of bed. Once the fractures are healed, most patients do well and so therapy not terribly useful at that time either. Deep breathing and even using an incentive spirometer to avoid an associated pneumonia can be helpful.