I am in Greece and have an active Playstation Plus account. In the blog post about PS3 Pex player availability, you include Greece in the countries were you can download the app. Although I can find it for PS4, I cannot find the PS3 version neither on the store nor on the "My Channels" menu as described a relative post.
I also have a PS3 which is in Turkey region. I have the same problem, too. Plex app is not listed on neither store nor my channels, however Turkey is listed in supported countries. I bought a Plex Pass subscription just to use it from PS3 and it is annoying not to use it.