I have read in several places that Nikon would replace a camera or lens after 3 failed repair attempts, but you won't find any official statement from Nikon to that effect, and there is surely always some discretion.
But in any case, I would first make sure the camera really has an issue. I.e. does it front focus with several lenses, and these lenses focus find on another body? That would be the easiest way to establish that. If you're sure it's the camera, I would send it in again, point to the 3 failed attempts and demand a replacement.