Epoxy - or most other high-strength bonding agents - really "cure" cracks, if correctly used, because it adds a cohesion and a strengthening layer to the shell. Works even better if we use some kind of fiber, whether it's a carbon or glass mesh or even chopped fibers to add to the resin as a compound. The fibers increase strength by a huge margin and weight less than the resin so less is needed to create a definite fix.
Only a few details are important in my experience: I've fixed a few shells for friends here using these various techniques, but 1) only at an early stage. If the shell is too weak or compromised (already damaged at various points or with big cracks), I'd discard it and get a new one; 2) I covered more area than the immediate cracks, because the forces that cause the cracks on crack-prone shells are part of the design and use (flying) of the P3, so they won't just go away and will "migrate" instead, to other areas and with different vectors and strength.
PS.: Regardless of all this it is always a good idea to reach out for DJI warranty in the first place!
