Would like mine please.

I should be clear that I haven't caught up on Season 3, but I've been keeping up with the show through others' reactions and commentary (and my own knowledge of the LNs which I have read). Anyway, you are pretty much spot on. Dai-Mankai no Shou is an anthology/"gap filling" season. It's basically showing the viewer what happened before and during Yuusha no Shou from both the Hero Club and the Sentinels' perspective. The Sentinels were most active between season 1 and s2, as that's when the KuMeYu LN takes place. But their story concludes in the epilogue chapter which takes place at the same time as YuYuYu s2's finale (Yuusha no Shou ep 5-6). KuMeYu generally exists for world building and to flesh out the events between the two seasons, and overall the adaptation seems to be well regarded by fans as it was a short novel. But it DOES skimp on characterization for some of the girls, especially Yumiko and Shizuku.

NoWaYu on the other hand, they ended up framing it as a flashback arc (the Hero Club reading Wakaba's diary during Yuusha no Shou episode 3, which is kind of how the novel's chapter 0 frames things as well although the exact timing in the series of when this happens is not specified) skipping the entire first half of the novel, so most of Wakaba's arc and basically the majority of Tamako and Anzu's scenes in the story were completely glossed over. It's a 19+ chapter light novel prequel and they ended up cramming in the last 7 or 8 chapters into 4 episodes. They basically only had time for Chikage and Takashima Yuuna's arc (which is also missing some details). They did all those aspects well, but still, you are basically missing an entire half of the story when watching season 3 but not reading NoWaYu.

Other than that, all the anime original content is just meant to expand on Yuusha no Shou. That works in that season's favor, but does no favor for anime onlys who didn't read the light novels. It's disappointing that the creators decided to do things this way, but fans got to see their favorite moments animated and finally get a more "complete" version of season 2 they can be satisfied with. This season was a better idea than trying to continue the show from Yuusha no Shou/s2's ending which was extremely definitive, but they probably should've had NoWaYu as its own separate thing so the Sentinels, Christian Era Heroes and the Hero Club would've gotten more even screentime. The final episode of this season is most likely going to be just an expanded epilogue of s2's finale which we didn't get originally, so that'll be nice.

So yeah, basically this season's a remake/expansion of s2 with content from the LNs providing people with context and lore that was missing from the first two seasons for a lot of anime only viewers. I love YuYuYu but its a franchise with a high barrier and I respect that, so if any of what I said clears things up for you and you feel like you'd like to get to know the characters from the LNs __better than you did in the show, I can't recommend NoWaYu and KuMeYu enough (also the Hero History Apocrypha series, which is three books that take place during and continuing from NoWaYu and flesh out several of the side characters, including Hinata and the Mikos in Taisha who were associated with the first generation Heroes).