Bad injury 2-3 weeks ago. Still cannot rotate arm palm down to palm up. Not that it hurts I just can't. Was diagnosed as elbow fx. Could this be tendon?

Maybe, but. Since you were diagnosed with a fracture, that certainly may be affecting your range of motion. Your dr should be following up with you to assure you are progressing normally. Orthopedics may be needed to assess your fracture care and physical therapy and rehab may be beneficial for improving range of motion. A physical exam would be able to help diagnose a possible tendon issue as well. Gd luck.