I was wondering if it is possible to do the upgrade of the environment directly from version 3 to 5, having only to upgrade the project itself through version 4. Can you please let me know if it is possible and for what issues I should keep an eye out?
Yes this is possible, your application database can convert from 3 to 5 just fine. Only the model needs an in-between conversion. There are no specific things to look out for, but as always, make a backup.