The DJI Phantom 3 drones are prone to stress cracks under the motors, much like this one over time. Its a faulty design however DJI will repair it if your drone is under warranty. If you start to see these stress cracks on your drone arms contact DJI for repair immediately. If you don’t have stress cracks on a Phantom 3, make sure its part of your regular flight inspection before flying.
The cause, and the solution
I believe DJI over tightens these screws, and the viberations from the motors and thin plastic weakens. All DJI Phantom 3 owners (even those without problems) should take a 2mm hex driver and back them off 30* and do the same for the two rear torx screws (T8 torx driver needed) as that area has been known to cracks as well.
Many people bought these arm mounts (including myself). I put them on to prevent any cracking, however I started to notice small hair line fractures under my mounts, so they do not preventing the cracks from forming, save your money and back off the screws off. After I backed them off I noticed the cracks stopped spreading. But once drone season ends I will be sending my drone back to DJI, they have about a month turn around time.
I don’t believe the Phantom 4 and above are affected.