Sorry by the Most of Arc 3 is not translated as of yet.
Meanwhile, credit for the translations of the Arc 3 Interludes belong to SummaryAnon, whose only known identity is his email address, ankaa.burner@gmail.com, as well as his Mega Archive. (This site is not affiliated with Summary Anon).
The anime adaptation left off at the end of Arc 3, without having adapted the interludes bridging Arc 3 and Arc 4; if you've just finished the anime, you'll want to start with those, then proceed to Arc 4. If you are interested in reading the Web Novel version of Arc 4 and beyond, consider checking our Cut Content page, you can find several important plot points from both the Web Novel and Light Novel that were skipped by Season 1 of the Anime; this will prevent confusion when some of this cut content is referenced.