Luxrender is an independent, open-source render engine designed to work with any number of programs which have plug-ins, or "bridge" as they're often called, written to connect them to it.  Plug-ins do not come with Luxrender.  They must be downloaded and installed separately as part of your program's core functionality.

For example, Blender has a plug-in called LuxBlend.  The one you mentioned, Pose2Lux, was written for Poser, and is a free alternative to Reality, though it lacks many of Reality's features.

Reality was developed three years ago, originally as an exlusive plug-in for DAZ Studio.  Reality 3 is the first and only commercially available version written specifically with Poser in mind, and is not part of Luxrender.  Nowhere in the Reality User Guide or any YouTube video is there any suggestion or implication to the contrary.

I clicked on the Poser icon on Luxrender's web site.  It clearly lists Reality as an optional plug-in for Poser in addition to Pose2Lux, and includes the direct link to the Reality 3 page at Pret-a-3D.

The latest version of Lux is 1.2.1.  That's the one you want regardless of which program you're going to use it with.