We know JJ liked the Knights of Ren (obviously, he created them), but I just don’t know if he’d pull them back in for the 3rd movie.
I’ve always headcanon’d (as most of the fandom) that the KoR were the fallen Jedi apprentices – TLJ seems to support that, as Luke mentions that Ben left ‘with a handful of students’. And we know that essentially they did errands for Kylo/Snoke – across the galaxy, gathering Jedi relics, maybe killing force users, etc. to eradicate them.
However we haven’t seen them outside of the forceback shot of Rey’s in TFA, there were maybe 5-6 of them and they didn’t really have lightsabers, they had all sorts of odd weapons.
It’s possible that the Praetorian Guards ended up being the KoR/Jedi students, who knows.
What I do know is that it is unlikely we see them in the film – I sense that we might get some explanation and backstory in a novel or comics perhaps – maybe even a TV show.
I think it’s far more likely that we’ll see a FO army that Kylo is struggling to maintain control of, and Hux possibly being a little rogue with them – striking places without permission, etc. which leads into a big conflict in the end.
So I don’t think it’s likely that we see KOR in the movies, or Rey joining them. I think it’s pretty clear that Rey is going to move the Jedi forward – but take them back to their roots. Which, as pointed out, are far more rooted in balance than we’ve seen them previously.