I keep saying it. You're not alone. Just like Phantom 2 times, we're here again with stress cracks. This time time with phantom 3. And where are the DJI guys? Avoiding this thread like the plague. It was no different during phantom 2 times and stress crack threads. In threads like these they can't use the textbook "were you using the latest firmware" line. If more folks looked over there they would find cracks. Considering I found 4 cracks in 8 of the side by side arm screw holes, the issue just might be far more prevalent on the Phantom 3. DJI should include crazy glue with the spare bag.
Anyway, forget warranty. Last I checked it's not covered. Unless there is a heads up, check for cracks announcement here in the forums. Which, needless to say, DJI won't be thrilled about cause they know this issue is real. This is why the Phantom 3 has additional shell retention clips.
First thing to do is speak up and I thank you for doing so. Hopefully others do as well. Next you have to make a decision if you want to make that part of your phantom stronger but also impossible to open without sacrificing a 100 buck shell. Look at it this way, it's not a flawless shell any more. It now sports standard DJI approved stress cracks. Your cracks are in the usual middle screw hole locations correct? If so, here is what you do. When holding the end of the arm and twisting it slightly you should see the crack open and close. If it's cracked in one side, the other side will crack soon due to that side now doing all the work of restraining those torsional stress forces. Holding your phantom on its side, run a bead of crazy glue along the upper and lower shell seam from the region of that screw hole over to near the small little end screw area. Goop it on there, no worries. Wait about ten seconds. Maybe less, maybe more but think about giving some time for that super glue getting in there in the seam. Now after those seconds, with a folded over paper towel, quickly wipe the excess in a quick swoop. There will be a trace but 400 grit will remove it. You will barely be able to see the results. Also do add closing pressure in the area. Use zip ties to help you with this. Once dry the torsional integrity of that arm will be restored and will be stronger since the whole end of the arm will take some of the brunt of keeping things together. Also add glue to the stress crack area. It will now not be doing 50% of the work, but it's nice to know it's doing something.
Am I happy with this repair? I don't mind now having a shell I cannot split. But I would be a lot happier if DJI would get on the ball and make phantoms stronger in that area. They added clips, changed the end screw location design and added clips. Couldn't they have thickened the plastic there? Something the far more powerful motors could easily handle when one considers the extra weight some folks were already adding to their phantoms in the form of trackers, gimbal guards and the like. But hell, right now I would be happy if DJI would stop avoiding these stress crack threads.
Good luck Michael.