written in the stars
What makes you (and other people as well) think they didn't do this already? I don't remember there being any trios in DDD, UX, or DR. In KH3, you can make a case for SDG, but other than that the concept has no relevance in it either. Also, some of the old ones got upgraded to quartets (RAXI, TAVC).
You are right. They did move away from it a bit except when they were resolving plot lines. They did add a new trip with Lea, Isa, and Subject X, though.