My favorite, and what I've used for a year now on my P2XL was the Maxboost mSnap Thin Cases Perfect Fit. Unfortunately they don't currently show anything for the P3.
My runner up was the Spigen Thin Fit. It didn't have the same full body protection as the Maxboost (sides were exposed) but I did like it a lot otherwise. Spigen does, apparently have the cases already, though they're not available on their website. They list 2 ThinFit options for the P3. A regular and a "360". Since I can't click on them I have no idea what the difference is. I WAS able to order one of these from Amazon. I'm assuming it's the regular one though. Figured it'll work until the Maxboost is released. Anyway, here's link: http://a.co/d/dcRtP8q