The primary change in V3 is the migration to Bootstrap V4, so any forked or added HTML templates will need to be redone. It looks like Code for Croatia has a ton of frontend HTML overrides, so instead of upgrading that instance, I’ve created a fresh new V3 staging instance for you: http://cfc-staging-v3.poplar.phl.io
I’ve cloned codeforcroatia.org’s live database to it, so you should be able to login to /develop with your credentials from there. Porting your overrides over one at a time so you can review/update them along the way would probably be the sanest approach
In this pass, we should try to eliminate you needing to override HTML templates so much. For missing translations, we want to add {_ "..."} wrappers so the string can be localized without overriding the template. For improvements/features we should merge those into laddr v3 if possible. Outside that, breaking out the laddr template to use an include for a spot we want to let people do their whole own thing with and inserting editable content blocks are two more tools in our arsenal to reduce how many templates you need to keep forked.
Let me know if you want this staging site’s DB synced up from live at any point