Openelec works great on the Pi2. I expect it would be excellent on the Pi3. I actually tried both Openelec and OSMC, and went with OSMC instead, but that pretty much comes down to personal preference only.
I have never tried Plexbmc, but it is my understanding that it works good. Here is a link with some good info regarding plex integration through Kodi using Plexbmc. Amber is a great skin for the Pi, and integrates Plexbmc very nicely.
http://www.htpcbeginner.com/kodi-and...plexbmc-addon/
Good luck