EP7 & 8’s manga have 9 volumes (some of them close to 300 pages iirc), maybe they want to do 4 volumes and went with the first 3 into one.
To be more precise, I think this was a bad move from them in a way:
Volume 1 is 356-pages long
Volume 2 and 3 are 240-ish
Volume 4 is 210-ish
Volume 5 is 220-ish
Volume 6 is 200-ish
Those 3 are much better candidates for a triple volume then the first 3, and end in a rather good cliffhanger too (much like volume 2 does).
And for the best candidate, we still have:
Volume 7 & 8 are 180-ish pages
Volume 9 is 220-ish pages long, and concludes the release.
Altenatively, volume 5-7 could be the most equilibrated triple volume, and ends in an acceptably good cliffhanger as well (volume 6’s is much better though).