Honestly, Nomura has a lot of room to work with when it comes to why Ephemer, Skuld, and Blaine aren't in the present timeline alongside Lauriam and Ventus. Like it was said many times, they went to another world to experience no tragedy, this could also be another realm where time flows differently or not at all. In KH3, Lea and Kairi are to be training in a realm that has no time, so the possibility is there. It's also a possibility that Ven and Lauriam escaped or were accidentally cast out of this realm and Ephemer, Skuld and Blaine are still in there which would explain their lack of appearance and allow them to remain as the same age as Ventus since it seems that they are the good guys and Lauriam is the baddie.
We also have no reason to believe that these characters will show up in KH3; that's the end of Xehanort's saga. If anything, KHUX could be setting us up for what happens afterwards. That being said, it's still too early to be seeking out placements for Larxene, Demyx and Luxord's real selves (hopefully I'm not speaking too soon ). They may be some added villains to the next saga.
Yeah, when you think about it, it's very interesting, because we got these new kids running the block, and you don't really know what's going to happen to them next. They're supposed to rebuild the world, but I'm betting something will go very wrong, otherwise why are Ventus and Lauriam so many years ahead and present in the current timeline? It's all mysterious, I tell you!
And well, you're right, these characters may not appear in Kingdom Hearts III, but maybe they could be mentioned in the form of reports or maybe some Easter Eggs or whatnot! As for Larxene, Demyx and Luxord's real selves, I think we'll see them in Kingdom Hearts III.