The issue of Final Fantasy characters being absent from Kingdom Hearts 3 has been touched on before. Both Nomura, the director of Kingdom Hearts, and Tai Yasue, the co-director of Kingdom Hearts 3, have provided answers to the question, but because Re: Mind officially releases tomorrow, Nomura has addressed the issue once more.
Tai Yasue claimed that Final Fantasy characters were absent for story reasons. The team wanted Kingdom Hearts 3 to focus solely on Kingdom Hearts characters, and they didn't expect fans to miss the Final Fantasy appearances that much. Nomura later claimed that they didn't appear because there wasn't enough time. Apparently, the team had the models and everything ready to go, but didn't have time to put them in the game.
Now, however, Nomura has given a definitive answer, and it's a combination of his and Yasue's. The information comes from a two-part QA series titled "7 Lights and 13 Darknesses." While handling the 13 Darknesses portion, Nomura received a question asking why Final Fantasy characters didn't appear in the original version of Kingdom Hearts 3. He replied that Kingdom Hearts 3's story didn't go in a direction that required a visit to Radiant Garden; that combined with work-related issues lead to the decision not to put Final Fantasy characters in the original story.
As for why the Final Fantasy cast has returned for Re: Mind, Nomura stats that Re: Mind's story requires them. He also notes that the passion from fans to see these characters again affected the decision to bring them back. The role Final Fantasy characters have in Re: Mind won't be a big one, but Re: Mind does at least give them a presence in Kingdom Hearts 3. From the looks of the trailer that announced their return, they'll be helping Riku and the others locate Sora to some extent.
The lack of Final Fantasy characters in Kingdom Hearts 3 was bizarre. But if fans really did sway the decision to bring them back, it's likely they won't be missing from another numbered Kingdom Hearts title (barring work-related issues, of course). Still, some reoccurring members are still missing. Cloud and Sephiroth are not confirmed to return with everyone else in Re: Mind. It's possible Sephiroth may come back as a surprise secret boss, and Cloud would likely accompany him if he does, but whether or not he will remains to be seen. Nomura claims Re: Mind will be the only DLC for Kingdom Hearts 3, so if the duo doesn't show here, they may not show at all.
Kingdom Hearts 3 is out now for PS4 and Xbox One.