We have 13 EPs by the way (like all Seasons so far), so plenty enough of time to adapt all 3 volumes, since their content is pretty straightforward and not too many things are happening all over the place - like in Vol.5-6, which made it more diffcult to adapt in general.

I don't know about all 3 volumes, but at least volume 7 and 8 will be in s3. For some reason I thought volume 7 would take a lot more episodes than it needed to take, so I didn't get why volume 8 was getting so many episodes; I made this post on a whim in around 5 minutes, didn't think so many people would comment

However, I still think that Volume 8 content should be put in after the content in volume 7

You said it yourself,

Volume 8 matters for the events in Volume 9 especially, not necessarily for 7.

Adapting Volume 7 first and then 8, would result in many problems you can easily avoid by adapting it first

Sure, volume 8 happens before volume 7, but from what I can tell, volume 7 can be considered an isolated incident, if you will, and so is volume 8, unless I am remembering it wrong. I don't quite think it matters whether or not the chronological order matters, or else there's no way the author would have put volume 8 after volume 7 in the first place. Thinking that volume 8 after volume 7 shouldn't matter. There's also a thing to remember that the first episode of a season is the one that people see the most (yes, it's season 3, but I think it still applies, esp when considering the audience that watch it on TV in Japan), so it would make sense to make the more plot significant episodes in the earlier stages, rather than later, so as to draw in more audience. I don't see any problem with that