I have linked Dropbox and got sync set up in Marvin 3, and it works great.

I'd like to know exactly where the Marvin 3 sandbox file is, but I cannot find it anywhere in my Dropbox. My Dropbox is practically empty, so I'm surprised I can't see it anywhere. It's not in the Apps folder so... ¯\_(ツ)_/¯

Where the heck is it?
Mine is in the Apps folder...but you may be able to put it in a different folder when you set it up??

\Dropbox\Apps\com.marvinapp\Sync
I think I figured out the issue. Marvin 2 used Dropbox for its sync location, but Marvin 3 uses iCloud for this. I think, anyway.
Yep, this. Marvin 3 sync is iCloud-based. Probably easier for majority of Apple users since that eliminates the need to sign up for a separate Dropbox account.

Iirc, Dropbox changed its API in 2017 breaking compatibility in older apps so I'm surprised people can still use syncing via Dropbox in Marvin 2.
I seem to recall that's why I switched from Marvin 2 to Marvin 3 (because the Dropbox integration was broken).
I broke my iPad Pro a month ago. My permanent replacement hasn't come in, so I haven't been able to access my iCloud files. Will all my Marvin 3 books along with their highlights and annotations be accessible once again? I backed my iPad regularly to the cloud, but I didn't use any in-app backup methods for Marvin3 specifically. I can access my "files" folder from my iPhone, but I don't see anything there.
iCloud backup is actually more complete than in-app Marvin backup. Marvin in-app backup only restores database, books, annotations, etc. iCloud backup restores pretty much everything including settings.